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Geology and ore deposits of the Yukon territory Freshwater, Norman G. Morgan 1930

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GEOLOGY AND ORE DEPOSITS OF THE YUKON TERRITORY A THESIS Presented for the Degree of Master of Arts in the Faculty of Arts and Science at The University of Br i t ish Columbia by Norman G. Morgan Freshwater. Apr i l , 1950. C O N T E N T S Introduction CHAPTER I Physiography CHAPTER I I General Geology CHAPTER I I I H is tor ica l Geology CHAPTER IV Descriptive Geology Pro-Cambrian Palaeozoic Mesozoic Tert iary Quaternary and Recent CHAPTER V Igneous Rocks CHAPTER VI Economic Geology BIBLIOGRAPHY Plate I Facing Page Plate I I Facing Page INTRODUCTION. In this thesis the wr i ter has attempted to g ive as complete a compilation of the geology o f Yukon Ter r i t o ry , as the l i t e ra ture permits. The geology o f ' Yukon Terr i tory i s lacking in sedimentary formations and those sedimentary formations present have a rather remarkable scarcity o f i d en t i f i ab l e f o s s i l remains. The work of W.B* Cockfield and E+ Lees during the f i e l d season o f 1929 promises to straighten out at l eas t some of the d i f f i c u l t i e s . What information the wr i te r has of these discoveries has been stated in the fo l lowing pages* A discussion o f the placer deposits of Yukon i s considered too extensive a subject to be dealt with in th is thesis and have thus purposely been omitted. Where possible and without going into too much de ta i l the wr i ter has attempted to corre late the geo-logy o f Yukon with that of the In t e r i o r o f Br i t ish Columbia. A complete bibliography dealing with the l i t e ra ture published on the geology of Yukon w i l l be found at the end o f this thes is . Ftcjt. "Ph\)Stogro.phtc. p3rovtuces of t h e Y u K o n Yukon t e r r i t o ry is essent ia l ly composed of three physiographic provinces which are continuous with similar div is ions in Br i t ish Columbia, to the southeast, and Alaska, to the west. These physio-graphic provinces named from southwest to northeast are: the Coastal system, the In ter io r system and the Rocky Mountain system. These three provinces form the Cordi l lera of North America. The Cordil lera extends from Mexico northward to the Arct ic Ocean, para l l e l ing , to an amazing degree, the Pac i f i c co^st l ine of North America. In Br i t ish Columbia the general trend uf the Cordi l lera is north-west, in Alaska the trend i s westerly, <vhile in "Yukon which is in between, the course of the Cordi l lera is intermediate between the two, generally nort?webtrrly. Lyin^ to the north, northeast, and east of the Rcc'.y Mountain system are various plains or lowland t r ac t s . - -The Arctic Slope region, the Mackenzie lowlanfs, areas of broken wode3 ^l^ins, possibly belonging to the great plains. ( See Fig 1 . ) ^ ^ A. The Coastal System. The Coastal system, from the v i c i n i t y of the 50th to near the 60th para l l e l , embraces only the (1) Cairnes D.D. — /Vhe t^cn D is t r i c t , Yukon t e r r i t o ry , G.S.C. Mem. 31, P. 9. Figure I I Nomenclature of the Mountains of Western Canada. (2) Coast range. The Islands to the west were considered (1) by Dawson as a separate range and are now ca l led the insular range by the Dominion Geographic Board.( See (2) F ig . 2) The simpl ic i ty of the Coastal system i s i n -terrupted near the head o f Lynn canal, whence northward nad northwestward this province embraces a number o f ranges or mountain groups including the Coast range, the St . El ias range, the Aleutian range and the Alaskan range, which are in places separated by wide va l l e ys . The Coast range consists, in a general way, of an i rregular complex of peaks and r idges, that possess but l i t t l e symmetry other than a rough alignment para l l e l to a northwesterly-trending ax is . The range has everywhere a precipitous and jagged aspect, and consists large ly of kn i f e - l i ke crests , rugged or even needle- l ike summits, and sharply incised va l l e ys . The summits in southern Br i t i sh Columbia, r ise to uniform al t i tudes of from 8,000 to 9,000 f e e t above sea- leve l , . but toward the north gradually decrease in e levat ion, unt i l in Yukon they stand at only 5,000 to 6,000 f e e t ; though the change in e levat ion i s apparently great , i t (ljDawson G.M. "On the l a t e r physiographic geology of the Rocky Mountain region in Canada, with special reference to changes in e levat ion and the history of the g l a c i a l Per iod;" Trans. Royal Soc. of Can. Vol. V I I I See. IV. 1890, P. 4. (2 ) Dominion Geographic Board o f Canada 1918. i s so gradual as not to break the general uniformity o f summit-level which, however, bears no re la t ion to structural features. The Coast range a f t e r fo l lowing the coast l ine from southern Br i t ish Colombia to near the head of Lynn canal, passes behind the St . El ias range, and thence northward, as f a r as i t extends in this d irec-t ion , constitutes the most easter ly d iv i s ion o f the Coastal system. North of Lynn canal, the Coast range gradually becomes less prominent, unt i l i t merges into the Yukon plateau in the v i c in i t y of Lake Kluane at o o ' la t i tude 61 and longitude 138 35. The Coast range has been considered by a (1) number of geo log is ts to represent a peneplanated or at least mature to old surface o f erosion. Other geo log is ts , however, maintain that this terrane shows no evidence of having ever been peneplanated. The St . El ias range as the name i s aoplied by (2) Brooks, with i t s broader s ign i f i cance , includes the Chugach, Kenai, and Skolai mountains which are oro-graphical ly a western extension o f the St . El las range as this term i s usually intended. Thus def ined, the St . El las range extends northwesterly from Cross sound, t l ) Dawson G.H. "Report on Kamloops map sheet, B.C." An. Rep. G.S.C. Vol . V I I . 1894 P.10B. Hayes, C.W. "An Expedition through the Yukon d i s t r i c t . " Nat. Geog.Nag. Vol . IV.P.128 (2).Brooks, A.E. "The geography and geology o f Alaska." U.S.G.S. Prof, paper #45 1906. bends westerly near the mouth of the Copper r i v e r , and near the head o f Prince William sound, in longitude o 147, turns sharply southwest and merges into the high-(1 ) lands of Eenai peninsula. The St* El las range var ies in width from 50 miles near Cross sound, to nearly 100 miles at Mount St . E l i as , and then narrows down to less than 20 miles to the southwest in the Eenai peninsula. This "range i s a rugged mountain mass para l l e l with and close to the Pac i f i c coast from Cross sound as f a r as the entrance to Cook i n l e t , with one spur, the Skolai mountains, stretching to the southwest. On the sea-ward aide, the range presents an abrupt escarpment, o f t en r is ing d i r ec t l y from the water, while i t s northern slopes almost everywhere, f a l l o f f abruptly to the Central Plateau." (Yukon Plateau.) Peaks in this range r i se to great e levat ions, f o r example Mt. S t . El las and Mt. Logan which attain an e levat ion above the sea of 18,024 and 19,500 f ee t respect ive ly . Skolai mountains, or more part icular ly the Skolai-Natazhat Mountain group, which are l imited to the north f o r a distance of approximately 40 miles by White River va l l e y , are rugged in character, with a l -t i tudes of 7,000 to 10,000 f e e t , and merge to the north or northwest with the Wrangell mountains. The most prominent break through the mountain barr ier or water-(1J Brooks, A.H. ^ Op. C l t . (5) shed comprised of Wrangell and Skolal mountains, i s known as Skolai pass, which l i e s between the heads of Nizina and White r i v e rs . Wrangell mountains owe the i r o r ig in to the ac-cumulation of volcanic material in times so recent that the forces of erosion have not yet removed i t ; and they thus^ d i f f e r from the other ranges of the Coastal system, which belong to that class of the earth 's features that are the result of d i f f e r e n t i a l erosion in regions of deformation and u p l i f t . The highlands of Wrangall mountains possess the i r regular forms typ ica l of v o l -canic mountains which are bu i l t up dominantly of lavas rather than ash deposits. These mountains are, a lso, exceedingly rugged end occupy an intermediate posi t ion between Skolal and Chugach mountains in the south and east , and Nutzotin mountains on the northeast. Mount Wrangell, with an e levat ion of 14,005 f e e t , i s central ly located in the group of mountains to which i t g ives i t s name, and a l -though possibly the most imposing among them, i t i s not the highest. These mountains reach the i r culminating point in Mount Sanford which i s 16,200 f e e t above sea-l e v e l ; and at l east f i v e other peaks r ise to elevations exceeding 12,000 f e e t . The higher port ions,of the Skolai-Wrangell ranges constitute a vast snow f i e l d from wi&eh scores of va l l ey g lac iers descend, and form (6) the headwater sources o f pract ica l ly a l l the more import-ant streams of the region. The Nutzotin mountains to the north, which are rea l ly an eastern d iv is ion of the Alaska range, are low-er , but on the whole do not appear to be less rugged than the Wrangell-Skolai mountains. The average e levat ion of the highest summits of the Nutzotin range i s from 6,000 to 9,000 f e e t , but th is range reaches i t s highest point in Mt. A l len, which has an e levat ion o f 10,420 f e e t . Nutzotin mountains, however, include no extensive snow f i e l d s and the i r few g lac iers are small and unimportant. B. Rocky Mountain System. The Rocky Mountain system extends northward from the western United States through Canada to near the Arc t i c , where, south o f Mackenzie bay i t turns a l -most at r ight angles, crosses the International Boundary, and continues in a d irect ion s l i gh t l y south of west, across Alaska to the ocean. South of Yukon this system i s notably complex and includes several high ranges whose axes are in general pa ra l l e l . The Roeky Mountain system o f Yukon has not been extensively explored and concerning i t r e l a t i v e l y l i t t l e i s known. This terrane, however, constitutes a mountainous be l t which stretches northward toward the Arct ic and forms in general the watershed between the Yukon r i v e r on the west and the Mackenzie r i v e r on the east . A f t e r bending to the south-(7) west and entering Alaska this system is known to be a complex mass and i s continued southwestward as a great trans-Alaskan chain to which the name Endicott mountains ( 1 ) has been applied* The Rocky mountains increase in ruggedness northward from the 49th para l l e l to Mt. Brown and Mt. Marehison where they atta in the i r maximum heights; from thence northward they decrease in ruggedness and e leva-t ion unt i l where the Peace r i v e r crosses they resemble only ro l l ing h i l l s . Their e levat ion var ies from a maximum o f 13,000 f e e t at Mt Robson to an e levat ion o f 6,000 f e e t where the Peace r i v e r crosses. They have an approximate width of 200 miles in the southern portion but the i r width decreases northwards. The Rocky moun-tains r i se steeply from the Great Plains on the i r east-ern border, and on the west they are bounded by the Rocky Mountain trench—- the master va l l ey of the Cor-d i l l e ras * The Mackenzie mountains, which include the greater part of the Rocky Mountain system in Yukon, are (2) described by Keele as a complex o f i r regular mountain masses which are the result of deformation and u p l i f t o f sedimentary rocks and include summits r is ing to t l ) Brooks A.H* ^The geography and geology of Alaska." U.S.G.S. Prop.paper #45,1906. PP 42-46 (2 ) Keele Joseph"A reconnaissance across the Mackenzie Mts. on the Pe l l y , Ross, and Gravel r i v e r s , TRikon and N.W. T e r r i t o r i e s . " G.S.C, 1910 PP. 16-18 (8) heights o f 7,000 to 8,000 f e e t above sea- l eve l . They have a maximum width o f about 300 miles. The Ogi lv ie mountains appear to constitute a northwesterly lobe o f (I) the Mackenzie mountains. The Mackenzie mountains are bordered on the west by a northward continuation of the In te r i o r system ( the Yukon Plateau } and on the east by the Mackenzie r i v e r and the Franklin mountains. The Mackenzie mountains are less rugged than the Rocky mountains and of greater l a t e ra l extent. Throughout Yukon the axis o f the ranges com-prising the Rocky mountain system, have an echelon character, the d i f f e r e n t ranges not persist ing any great distance, but instead g iv ing place in e i ther d i rect ion to other para l l e l ranges. North of the Yukon r i ver and just south of the (2) Porcupine r i ve r Cairns describes the Keele mountains as having undoubtedly had the seme physiographic history as the surrounding portion of the Yukon plateau and they are considered to constitute a portion of this terrene . " The Eeele mountains are characterized by deep canyon-l i ke va l l eys , and the included, i n t e r - va l l e y , p la in-l i k e upland areas* The rugged, craggy, steeply inc l in -ed va l ley walls r ise abruptly f o r 1,500 f e e t or more to (1 ) Cairns D-D. "The Yukon-Alaska International Bound-ary, between Porcupine and "Yukon r ivers Mem. 67. G.S.C. 1914 P.23. (2)Cairns D.D. Op. c i t . P. 26. (9) The upland above, and viewed from the edge of one of these inc is ion- l ike drainage ways, the topography appears to be rugged in the extreme. When viewed from the up-land, wel l back from the va l ley wal ls , a p la in- l ike sur-face i s presented with only occasional small, wel l rounder summits r is ing above the general l e v e l . Such a nearly base- level led or peneplanated and typ ica l l y old topography was apparently produced when the region stood much nearer sea- leve l than at present. In accord-ance with this assumption the p^anating process must have been^interrupted by a regional u p l i f t while occas-ional h i l l s s t i l l remained to re l i eve the monotony of the former landscape, and these now constitute monad-nocks or residuals r is ing above the general plateau surface. The Ogi lv ie Mountains are unusual in the fac t that they are composed essent ia l ly of so f t sedimentary rocks - - - mainly limestone. The va l l eys and lowlands are composed of s lates and schists which general ly are more resistant than limestones* In Yukon the extremes of temperature have more e f f e c t than the usual forms of erosion* The slates and schists permit ready access of water,and f ros t action i s prevalent to a high degree, thus the slates and schists sp l i t along the cleavage planes and readi ly break o f f . The f ine-grained stone, on the other hand, does not permit the entrance (10) of water to any great extent therefore f ros t -ac t ion on the limestones i s not as great on the s lates and schists . Thus the limestone forms the tops o f the mountain ranges in th is v i c i n i t y . C. The In t e r i o r System* The In te r io r system of Yukon, the Yukon Plateau, la a continuation o f the in t e r i o r plateau region of central Br i t i sh Columbia. I t i s cut o f f from the in-t e r i o r o f Br i t ish Columbia by the Cassiar range which o occurs in the v i c i n i t y of lat i tude 58 The Yukon plateau, comprises the major portion o f the Yukon t e r r i t o r y and i s drained almost wholly by o the Yukon r i v e r . I t extends from about la t i tude 58 in northern Br i t i sh Columbia, through Yukon and Alaska to the Bering sea, and has a width o f from 200 - 400 mi les, stretching from the ranges o f the Rocky Mountain system to the inner members o f the Coastal system which f r inges the Pac i f i c ocean* Into the upland surface of th is plateau pro-vince in Yukon, the main drainage courses have incised channels varying from 5,000 to 4,000 f e e t in depth, thus producing a very i r regular topography consisting essent-i a l l y o f r o l l i ng uplands separated from each other by wide deep va l l ey trenches. The summits o f the unreduced h i l l s and r idges, ly ing between the waterways, const i t -ute remnants o f what was once, apparently, a gently r o l l -(11) ing plain sloping toward the northwest. The plateau seen from a summit having an e levat ion corresponding to that of i t s surface and where one of the major r i v e r trenches does not cut the l ine o f sight displays an even sky-l ine sweeping o f f to the horizon, and broken only here and there by i so la ted , residuary masses r i s -ing above the general l e ve l * This upland, however, bears no re lat ion to rock structure, erosion having bevel led the upturned edges of the hard rock as wel l as the so f t s trata, with the result that i t s surface i s ent i re ly discordant to the structure of the highly contorted, metamorphie rocks that outerop over i t so extensive ly . Along the northern portion o f the Coast range, the general summit-level merges into that of the Yukon plateau, in a manner suggesting the synchronous plana-(1 ) t ion o f these two provinces, but during the various v e r t i ca l movements that have a f fec ted these terranes, the u p l i f t has been greatest along the axis of the Coast range and least along that of the Yukon plateau, which terrane i s thus given the contour of a huge f low-ing trough whose median l ine i s in a general wa^,mark-ed by the present posi t ion of the Yukon f i v e r from near i t s headwaters in northern Br i t ish Columbia to Bering (1 ) Speneer A.C. Bul l . Geol. Soc. o f America. Vol . XIV. P. 132. Brooks, A.H. "Geography and Geology of Alaska." Prof . Paper, #45 PP 286-290 U.S.G.S. (12) Sea* No d is t inc t l ine o f demarcation indicates the boundary between the plateau and mountain prlv inces; these, instead, grade into each other, so that a tran-s i t i on be l t occurs, generally from 1 to 4 miles wide, in which many o f the points cannot d e f i n i t e l y be said ( 1 ) to belong to e i ther terrane* The plateau topography i s characterized by two str ik ing features, the numerous, i r regular ly-d is tr ibuted wide, deep, steep-walled va l l e ys , and the elevated and in places s l ight ly,undulat ing, in t e rva l l ey , upland areas. Over considerable portions, the plateau surface has been almost i f not quite destroyed by l a t e r erosion, and in such places the topography consists of i r regular-ly -d is t r ibuted , rounded h i l l s , many of them gent ly-contoured and with summits that are in many cases remark ably uniform in e levat ion. Towards the south and west the surface o f the Yukon plateau gradually r ises and becomes more and more dissected, and the topography consequently assumes an increasingly rugged aspect unt i l the Coast range i s reached. The topography of the Yukon plateau may be divided into two groups;- the Uplands and the Val leys. (1 ) Cairns D.D. "Att in d i s t r i c t , B.C." G.S.C. Mem. #37, 1913. P. 16. (13) The Uplands. Over large portions of the Yukon Plateau region, the plateau character ist ics are s t i l l wel l pre-served, and numerous fragments of almost f l a t , or gent-l y ro l l ing upland s t i l l remain in spite of sub-aerial erosive agencies which tend to destroy the old surface. Over considerable portions o f the plateau region, l i t t l e trace of the former upland remains and the topography consists o f i so la ted , generally rounded, h i l l s , many o f the summits o f which r ise to an e levat ion of the plateau surface. In places erosion has succeeded in re-moving the plateau surface en t i r e l y , and only low, i r -regular h i l l s remain which show no concordance of sum-mit l e v e l . The plateau surface to be observed to the best advantage mast be viewed from interstream points, s i t -uated some distance back from the edges of the master va l l eys ; from such posit ions the even, gently ro l l ing character of the plateau i s s t r ik ing ly apparent. The surface bears no re lat ion to rock structure, the various rocks underlying the plateau are a l l truncated, regard-less o f the i r structure, hardness, composition or other qua l i t i es . The plateau-surface thus represents a plain of erosion that has probably been produced mainly by ordinary erosive agencies rather than by g lac ia t lon , (14) since i f i t has been produced by g lac la t ion i t s sur-face would be everywhere strewn with fore ign g l ac i a l materials, which i s , genera l ly , not the case. In places there i s evidence such as the occurrence of s tr iae and e r ra t i cs , that i ce has moved over the l oca l plateau surface, but the greater part of the upland i s covered with l o ca l material produced by ordinary erosive and weathering agencies. This plateau surface thus appears to form part of a region that during a long period of crustal s tab i l -i t y was almost completely ba^e- leve l led and was reduced to a condition of o ld age. The residual mountains that now constitute monadnocks r is ing above the plateau l e v e l , represent the only considerable elevations that remained to break the monotony o f the former landscape. Base- leve l l ing processes, which tended to reduce the ent i re plateau region to sea- leve l were interrupted, be-fore the reduction of these remaining h i l l s , by an up-l i f t which a f f ec ted a great portion, at l eas t , of Br i t ish Columbia and Yukon. The Yukon plateau province has been studied by a number o f geo log ica l observers, among whom there i s a consensus of opinion that i t represents a region which during a long period of crustal s t ab i l i t y became almost completely base- level led and was reduced to a state of old age. Accordingly at one time this region ( i s ; East have formed a portion o f a p la in, the edge of which was at or nearly at sea - l eve l . This base-l e v e l l i n g process was followed by a widespread u p l i f t and the nearly f l a t or gently undulating lowland became an upland t rac t . This u p l i f t rejuvinated the streams which immediately commenced trenching the i r va l leys in the upland surface and a new physiographic cycle was inaugurated. There i s some d i f f erence of opinion as to the exact date of th is planation and subsequent up-l i f t , but the bulk o f the evidence goes to show that th is region was pinnated during e i ther the Eocene or Pre-Pliocene Post-Eocene time, and that the planated tract was up l i f t ed to nearly i t s present posit ion during l a t e Miocene, Pl iocene, or nearly Pleistocene (1 ) time. The amount o f u p l i f t i s somewhat i nde f i n i t e . The Lewes and Yukon r ivers have grades much in excess of r ivers traversing a d i s t r i c t in i t s o ld age. Further i t seems very improbable that the area, pr ior to u p l i f t , was drained by a longer water system than the present circuitous one; in f a c t , invest igat ions have tended to show that the d i s t r i c t was drained into (2) the Pac i f i c by a much shorter system. The v e r t i c a l extent of the u p l i f t was probably in the neighbourhood (1 ) Cairns D.D. "Wheaton d i s t r i c t , Yukon Te r r i t o r y . " G.S.C. Mem.#31, 1912 PP 83-84 (2 ) Brooks A.H. "Geography and geology Of Alaska," Prof . Paper #45 U.S.G.S. 19Q6 P. 294. (16) ( 1 ) o f 4500 f e e t . Sometime subsequent to the u p l i f t , and the de-velopement of the present va l l ey systems, a cl imatic change caused g lac iers to form in the higher regions, and great tongues of i ce moved from the gathering groun-ds down the main va l l eys of the plateau and the uplands of the plateau surface were only s l i gh t l y a f f e c t ed . (2) Cockfield i s o f the opinion that g lac iat ion as an i ce -sheet did net e f f e c t the Yukon plateau. He bel ieves that the Coast range and the Mackenzie mountains acted as accumulating centres f o r the i c e which eventually descended upon the plateau in tongues fo l lowing the main r iver va l l eys and to a small extent covering some of the upland portions of the plateau-surface. These ice-tongues deposited morainal material in the va l leys and on some o f the upland surfaces; and in some eases formed dams in the streams due to the rapid retreat of the i c e , thus also forming lakes. I t has bean calculated that when the Cordil leran iee-sheet was present in North America the snow l ine was lowered 3,000 f e e t . In Yukon the snow l e v e l i s 7, 500 f e e t and a lowering of 3,000 f ee t would place the snow l ine at 4,500 f e a t . The general l e v e l of the plateau i s approximately 5,000 f e e t , thus the snow l ine (1 )Caims D.D. "Att in d i s t r i c t , B.C." G.S.C. Mem.37, 1913 P.23. ( 2 ) Cockfield W.B. Personal Communication. (17) during the time o f the Cordil leran ice-sheet would be wel l below the general l e v e l o f the plateau. The accept-ed reason f o r the absence of g lac ia t ion in Yukon i s the lack o f precipi tat ion* The Yukon plateau i s a dry b e l t , which i s readily rea l i zed by the volume of water in the drainage systems. The Yukon r i ve r drainage basin i s as large as that o f the St . Lawrence r i ve r system but the volume of water i s only one seventh. Thus i f , during Cordil leran iee-sheet time, the moisture laden winds came from the Pac i f i c in a southwesterly d i rec t ion , as bel ieved, the St . El las range and the Coast range would prove an e f f i c i e n t barr ier to prec ip i tat ion over the Yukon Plateau region. At no time, apparently, did snow gather on the plateau in su f f i c i en t quantit ies to form consider-able masses of i c e , bat f o r the most part, i t seems to have been blown by the winds into the va l leys and de-pressions. I t i s bel ieved that, during Pleistocene and Recent times, the plateau surface, although only modified s l i ght ly by moving i c e , has been considerably a f fec ted by accumulations of snow. The e f f e c t s o f neve snow are to eonvert shallow V-shaped va l leys into f l a t U-shaped depressions, to e f f ace the i r drainage l ines without material ly chang-ing the i r grades, and in this manner to produce general smoothness of surface. Since the snow-drifts have no (18) sl id ing motion, there i s no transportation of material by them; however, because of excessive f r o s t action, and continued alternations of f reez ing and thawing, the rocks at the peripheries of the quiescent snow, are f i n e l y comminuted and the material i s removed by in-numerable r i l l s to neighbouring depressions* These ( 1 ) e f f e c t s o f the work of quiescent neve, cal led nivat ion, have resulted in grading, to a considerable extent, the already gently ro l l ing surface of the plateau region, and account f o r the great amount of f i ne material that f i l l s a l l the minor depressions in the upland surface. The presence of the snow also helped to preserve the smooth outl ines of the topography, by protecting the surfaces from stream act ion. A review of the above shows that the fo l lowing causes mainly account f o r the contrasting topographies of the Coast range and Yukon plateau:-1. The Coast range was up l i f t ed :aore than the plateau tract and was consequently subjected to a greater degree to erosive agenciesg and as the moun-tains of the Coast range are composed mainly of homo-geneous grano-diorite the forms produced by erosion are noticeably i r regular since no bedding planes or l ines of hard and so f t layers ex ist to be emphasized by de-gradation. t l } Cairns D.D. "A t l in d i s t r i c t , B.C." G.S.C. Mem. 3V, 1913 P. 25. (25) 2. The rocks of the Coast range are general ly harder and mere resistant to ordinary sub-aerial agen-c ies than are the rocks to the east* and the more near-l y the rocks o f the plateau approach those of the moun-tains in physical properties the less apparent, and the more gradual I s the change from plateau to mountain provinces. 3. The Coast range i s su f f i c i en t l y hign to s t i l l hold great amounts o f g l a c i a l i ce which i s act ive ly employed accentuating the features of the mountains and g iv ing them a typ ica l f r e t t ed appearance. In the ease o f the Yukon plateau, on the other hand, the i c e , except f o r small masses in occasional cirques, has long sinee vanished from the region, and instead of the features there continuing to become more pronounced, they are being rounded and smoothed ever by n ivat ion. Thus once a d i f f e rence of e levat ion between these two provinces was established, the i r features became continually more and more contrasted. This aptears to account mainly f o r the str ik ing d i f f e rence in the physiography o f the terranes, although apparently synchronously planated ( 1 ) and up l i f t ed* The Val leys. A f t e r having considered the gently undulatory character of the upland, the observer w i l l be impressed (1 ) Cairns D.D. Op. c i t . P. 25. by the praaaameed topographic unconformities that pre-sent themselves at the contacts o f this surface with the high, steeply- incl ined walls o f the various va l leys t i l that intersect i t * The l as t great u p l i f t , probably in Pliocene time, which a f f ec ted this d i s t r i c t , g ive the streams of the region renewed l i f e and energy, and they im-mediately began vigorously trenching their channels in the up l i f t ed surface* Throughout the area deep inc i s -ions were rapidly made, which, in Pleistocene time, were invadad by g lac iers from the mountains to the west aàd south. Although the ice produced but l i t t l e e f f e c t upon the upland surface, i t had a profound influence upon the va l leys* When a broad ice-sheet covers a d i s t r i c t i t has the e f f e c t o f moderating the topographic features and ^educing the r e l i e f , by eroding material from the higher elevations and depositing i t in the depression. However, where the ice occupies only the va l l e ys , such greater end d i f f e r en t results are ss-en. Bare the interstream areas maintain the i r even character, un-a f fec ted by the i c e , xhlle the va l leys are widened and deepened; and the maximum a f f e c t s of g lac ia t ion are produced in areas where tihe topography indicates that TH"Cairns D.D. "Wh&aton d i s t r i c t^ Yukon Territory^""** G.8.C. Mem. 31, 1912 P. 16. (21) 1$ has been previously prepared to receive the i c e , by having deep va l l eys already made, in which the ice can moat advantageously operate. The V-shaped va l l eys are then transformed into wide, deep, steep-walled, U-shaped depressions, and hanging va l l eys , cirques, roches mou-tonnes, and other wel l known g l ac i a l forms are produced* In addition to being mainly destruct ive , the g lac i e rs also acted in a constructive capacity, and con-tributed vast amounts of morainal and other materials which deeply covered the f l oo rs of the master-depress-ions. Since the retreat of the i ce but l i t t l e erosion has occurred, and considerable portions of these va l l eys are s t i l l almost as the ice l e f t them, and the larger the va l l eys the more material they have received. The broad, f lat-bottomed, steep-walled va l l eys are found only in southern Yukon and in the v i c i n i t y o f the mountain ranges, which as noted be fore , acted as co l l ec t ing regions f o r the l e e . Northward, wide, f l a t , gently sloping va l l ey walls with interlocking spurs are more common. These are the unglaciated va l l eys of the plateau province. The g lac iated va l l eys are usually dotted with numerous lakes and muskegs bat the non-g lac iated va l l eys have a complete drainage system. The small streams that traverse the apland f low over th is sarface in wide, f l a r ing depressions, with gentle gradients, but on coming to the edge of this (32) elevated platform they plunge suddenly, by successive f a l l s , through gorge-shaped incis ions, to jo in the master streams below. Thus the tr ibutary streams have hanging va l l e ys , and the smaller the tr ibutary , the l ess eroding power of the stream, e i ther of ice or water, which i t contained, and the more i t s va l l ey was l e f t hanging abcve that of the master stream. Per fec t ly developed and well preserved crlques occur p l e n t i f u l l y , and ex ist prevai l ing ly along the edges of the master-val leys. A notable feature in con-nection with these forms i s that they are invariably found along the sides of the larger va l l eys , or at what may be considered the heads of the almost ins ign i f i cant sub-tr ibutaries. The cirques that o r i g ina l l y existed at the heads of the larger t r ibutar ies have, in a l l cases noted, been successful in gnawing headward to meet others moving in an opposite d i rec t ion, and cols or (1 ) passes have resulted. Glaciation has generally extended pract i ca l l y to the top of the cirque-walls, but above and back of this only nivation has been operat ive. The snowbanks are always the forerunners of g l ac i e r s , and when they accumulate on favourable h i l l s i d e s the comminuted mater-i a l produced i s removed, not as in the case of d r i f t s (1 ) Cairns D.D. "Wheaton d i s t r i c t , "Yukon Te r r i t o r y , " G.S.C. Mem. 31. 1912 P. 20. (23) on the upland surface to f i l l miner depressions nearly, and so to moderate the topographic r e l i e f , but instead innumerable r i l l s carry the f i n e l y broken material to Rarggr waterways which distr ibute i t widely* As this continues, the d r i f t s enlarge the i r containing nooks, and nourishing catchment basins f o r g l a c i a l ice are soon produced, and in th is way many cirques are formed. The boundary between the quiescent neve snow and the g l ac i e r is)marked in cirques by the wel l known bergschrund, or semicircular, i r regular crack that occurs approximately conforming to the contour o f the cirque, but standing somaidistance from i t s walls* gatthes has calculated that the neve mast be 125 f e e t thick and on a 12 per ( 1 ) cent grade before motion commences. From the immediately preceding statement i t would seem, since the general l e v e l o f the va l l ey f l o o r s i s roughly 1,500 f e e t below the plateau l e v e l , that a l l the va l l eys o f the Yukon plateau should have been g la -c iated. This i s stated in view o f the f ac t that the snow of the uplands was considered as blown into the depressions, therefore the va l l eys should have been f i l l -ed with roughly 1,500 f e e t o f snow and the va l l ey walls having a grade of greater than 12 per cent would produce motion* There appears to be only one reason why th is (1 ) Caims D.D. "Wheaton d i s t r i c t , Yukon Territory.* ' 6.S.C. Mem 31. 1912 P. 20. (24) neve did not form and th is i s that there was not s u f f i c -ient snow, or there was not 125 f e e t of more in the va l l eys . As explained be fore , the Yukon plateau i s a dry be l t between two mountain systems and the absence of g lac iat ion in the northern portion of the plateau shows that th is area was, as now, a dry b e l t . The moun-tain system on the west protected the in t e r i o r regions from prec ip i tat ion but formed the gathering ground f o r the va l l ey g lac ie rs which sent out the i r tongues of ice onto the plateau region. I t i s the b e l i e f of the wr i ter that too much erosive power i s attributed to g lac ia t i on , in the lower l e v e l s espec ia l l y . Glaciat ion, in the wr i t e r ' s opinion, i s more of a moulding, somewhat scourging, process in w&lch the sharp l ines o f non-glavial erosion are e f faced . I t i s probable, too, that the erosive power of g lac ia -t ion in the higher l e ve l s may be due to f r o s t action and the extremes of temperature. These conclusions are engendered by the wr i t e r ' s l imited experience with g la -c iat ion and w i l l be cheerful ly recal led in favour of evidence to the contrary. In a number o f places in the va l leys terraces are found, which are o f ten quite persistent and ex ist up to 700 or 800 f e e t above the va l l ey bottoms. These are peculiar in that, although from a distance, pre fer-ably from the other side of the va l l ey in which they (35) occur, they may be quite evident and even somewhat s tr ik ing on account o f the i r persistency, s t i l l when a person i s actually on the spot where they occur? i t i s o f ten d i f f i c u l t to t e l l exactly where they l i e , since only s l i ght amounts o f terrace-tops or platforms (1) remain. Generally several of these terraces can be seen cl inging to the va l l ey -wa l l s , and in a few places four of f i v e may ex i s t , one above the other. Of these, the majority extend but a short distance. These terra-( 2 ) ces have been described by a number of wr i ters ; and everywhere the o r i g in of these terraces i s in doubt. Dawson and Spurr consider that subsequent to the u p l i f t o f the Yukon plateau, and a f t e r the va l l eys had become deeply trenched, a submergence occurred in la te Pliocene or Pleistocenetime. The va l l eys are thus thought to have become part ly f i l l e d with grave ls , sands, s i l t s etc* A f t e r a b r i e f period, e levat ion commenced, and as the streams cat down through the debris terraces were l e f t cl inging to the va l l ey w a l l s - - the amounts ( i ? Cairns D.D. "Wheaton d i s t r i c t , Yukon Te r r i t o r y , " G.S.C. Mam 31. 1912 p. 21 ( 2 ) Dawson G.M. "Trans. Roy. Soc. Can. Vol V I I I , Sec 4, 1890, PP. 36-41, 48, 49. McConnell R.G."An.Rep.Geol.Snrv. Can. Vol.IV,1888-89. Russell j . c . "Bull .Geol.Sec, o f Amer. Vol.1,P.139. Spurr J.B* "Geology of the Yukon Gold d i s t r i c t , " 18th An.Rep. U.S.G.S. Pt . I l l ,1896-97 PP. 268-69. Sordenskjold O.The American Geol. Vol.XXIII.PP.290-98 Brooks A.H. "Prof.Paper #45,1906,9.S.G.S. P.296. (26) of the subsidence and u p l i f t being indicated by the terraces* The postulation of a submergence and subsequent ( 1 ) u p l i f t appears to Caimes to be quite uncalled f o r , to explain the or ig in of these terraces. Quoting Caimes; -"Tt i s true that a certain amount of u p l i f t has occurr-i ed in recent times and may be s t i l l in progress, as in-dicated by recent rock terraces along the Yukon r i ve r above Dawson and elsewhere, but these appear to have had an or i g in quite d i s t inc t from the grave l , sand and s i l t terraces which characterize many of the va l l eys of northern Br i t ish Columbia and Yukon. In whatever manner the terraces were formed, they must have or ig inated since the g l a c i a l period, otherwise the va l l ey g lac iers would have ent i re ly ob l i terated them. I t i s further evident that no great amounts of material have been deposited in many of the va l leys since g l a c i a l time, as in depress-ions such as along the White Pass and Yukon railway between Corcross and Whitehorse, and around Annie lake, the va l l ey f l o o r i s pot-holed and minutely rough, and possesses s t i l l the character ist ic appearance of a sur-face that has been overlain by i c e . " Brocks and others have supposed the terraces to be due to changes in the erosive powers of the stream, and in places this appears to be true, but in the per-i l ) Caimes D.D. Op. c i t . P.22. (27) t ion of Yukon t e r r i t o ry where the terraces reach high up on the va l l ey wal ls , th is theory ca l l s f o r the f o r -mer existence o f vast amounts o f material over the pre-sent va l l ey f l o o r s , which cannot be true in some loca l -i t i e s * I t has been supposed that the terraces are remnants of l a t e r a l moraines formed along the edges of the va l l ey g lac i e rs , and consist thus part ly o f ground-up debris accumulated by the ice i t s e l f , and part ly of materials that ro l l ed down the s idahi l l s from above, and gathered along the Upper surface of the i c e . As the ice retreated, and stood at successively lower e levat ions, ether accumulations would tend to form, and those l e f t above would remain in the form of terraces cl inging to the va l l ey walls* The most persistent and prominent o f the terraces would thus mark elevations at which the ice maintained constant elevations f o r except-ional ly long periods. In certain va l leys where the terraces have been but poorly preserved i t i s d i f f i c u l t to disprove this theory. However, in some cases, quite extensive f l a t -topped terraee accumulations remain in the meuths of the t r ibutar ies , and extend out f lush with the edge of t h e ^ a l l s of the master-val ley. I f the terraces or ig in-ated due to l ee action, the ice would also have invaded the mouths of the t r ibutar ies , and the ent ire lower (28) portions of such would not now contain f lat- topped accumulations. I t thus seems evident, as suggested by Norden-skjold and other, that these terraces are dominately, at l e a s t , lake terraces, and represent successive e leva-tions at which the water stood in post -g lac ia l time. This ca l l s f o r a damming o f the drainage system some-where along the Yukon r i v e r . As the terraces indicate that the period o f submergence was only b r i e f , the damming was probably due to accumulations of i ce or other g l a c i a l materials. The great masses of i ce which occupied the master-depressions planated the va l l ey s ides, reducing a l l project ing spurs, r idges, e t c , and bringing them into alignment to form quite regular wal ls . Since the close o f the g l a e i a l epoch, the numerous small tr ibutary streams from the upland have been cutting channels in these walls and enlarging the pre -g lac ia l Incisions in them. The result i s that, cat in these s teep ly- inc l in-ed va l l ey slopes are numerous V-shaped, trench-l ike fOrms, and between these are facetted forms carved in the va l l ey -wa l l s . These forms are quite pronounced along the g lac iated va l leys and represent forms produc-ed by a combination o f g l a c i a l and post -g lac ia l a c t i v i t -i e s . The stractual control o f drainage courses and (89) A the e f f e e t of rook structure upon topography in Yukon are questions which have not been dealt with at length (1) by geolog ists and ethers writ ing of the region. Cairnes states that the present course of Yukon r i ve r marks the axis of the Yukon plateau^ towards which there i s a gradual slope both from the east, and that this was ( 2 ) caused by d i f f e r e n t i a l u p l i f t . Cockfield and Be l l are of the opinion that a number of the larger va l leys in Whitehorse d i s t r i c t l ikewise had the i r posit ion deter-mined to a great extent by structural f ac to rs . They also noted a rather str iking rectangular e f f e c t in the drainage of the McClintock basin, which suggest struc-tural control o f drainage* (3 ) Reinecke noted the re lat ion of drainage to structure in the In t e r i o r plateau of Br i t ish Columbia* There, he found the general trend of the drainage to be north, northwest and northeast and also noted a marked r e c t i l i n ea r i t y in drainage pattern. Reinecke i s o f the opinion that the drainage i s control led by fractures (4 ) trending north northwest and northeast. Schof le ld f inds that the fracture system of Br i t ish Columbia has three main direct ions north, northwest and northeast. (1 )Caimes D.D. "wEeaton d i s t r i c t , Yukon Te r r i t o r y . " G.S.C. Mem 31 PP. 84. 1912. (2 )Cockf ie ld W.E. & Bel l A.H. "Whitehorse d i s t r i c t , Yukon." G.S.C. Mem. 150 P.5. 1926. (3 ) Reinecke L . "Physiography of the Beaverdell-Map Area and the Southern Interior."G.,S.C. Mus. Bul l . 11 1915. (4 ) Schofield S.J. G.I.M.M. - - Bul l . (30) Although thero I s l i t t l e information on the fracture system o f the Yukon t e r r i t o r y , from a study of the drainage pattern there appears to be a de f in i t e de-sign. Most o f the streams have a general north, north-west or northeast trend and some r e c t l l i n ea r i t y i s noted. I t thus seems probable and possible that the fracture system o f Br i t ish Columbia may be extended into Yukon. As has been shewn, g lae ia t i cn has modified the va l l eys of the streams and has l e f t deposits of morain-a l material upon the va l l ey f l o o r s ; but g lae ia t ion has also modified the drainage* On the wide, f l a t va l l e y -bottoms of the g lac iated va l leys innumerable pot-hole lakes are found. These lakes are due part ly to the scouring action o f the ice bat mainly to the disruption of the o r i g ina l drainage by damming with g l a c i a l debris, leaving a disconnected drainage system. Through the g l a c i a l debris the r ivers wend a rather sinuoas course and some lakes are formed by the cutting o f f o f ox-bows. In quite a number o f eases, the drainage has been reverted by the damming of the ancient va l l ey With morainal material and, as in the ease of Miles canyon near Whitehorse, with recent f lows. An inspection of the map suggests certain rather obvious changes that may have taken place. Lewes r i ve r may have flowed through the lower part of the va l l ey of Watson r i ve r Instead of around by lake Bennett. Wheaton r i v e r may have fol lowed i t s same course above Big Bend and here turned to the l e f t instead of to the r i ght , f lowing northward through the va l l ey now occupied by (1 ) f Annie lake. Cockfield and Bel l are o f the opinion that changes such as these were due perhaps, to one or both o f two fac tors : 1. deposition of g l a c i a l materials in the former va l l eys and 2. diastrophic movements. (2) Caimes suggests that the Lewes r i v e r formerly flowed through the Ogi lv ie va l l e y , which i s a continuation o f that occupied by lake Laberge, and which channel has been f i l l e d with morainic and other d r i f t material dur-ing the g l a c i a l period, forcing the r i v e r to seek i t s present course. (3 ) The evidence in Upper White River d i s t r i c t would seem to indicate that Upper White r i v e r in pre-Glacial time fol lowed a course similar to that of ^he present stream as f a r as the International Boundary, but below that point, this r i v e r swung to the north and continued down through Lake Tchawsahonon va l l ey to the Beaver. The Genere r i v e r , on the other hand, apparently held pract ica l ly i t s present course to the White River v a l l e y , and thence persisted as now, through the Nutzotin mountains, was joined by Koldem r i v e r and (1 ) Cockfield W.R. and hel l , A.R. "Whitehorse d i s t r i c t , Yukon.* G.S.C. Hem 150 P.5 1926. ( 2 ) Caimes D.D. "Lewes and Nordenskiold r i v e rs , Yukon." G.S.C. Mem.5 P.15 1910. (3 ) Caimes D.D. "Upper White River d i s t r i c t , Yukon." G.S.C. Mem. 50 P.61 1915. (38) other streams and was united with the Upper White r i v e r which had been joined by the waters of the Upper Tanana. These combined streams then may have flowed e i ther down the Tanana and thence to the Yukon, or may have continu-ed down the present va l l ey of the White to the Donjek r i v e r a decided change occurs. The va l l ey o f the White above this point i s wide and contains on e i ther side heavy banks and terraces of g l a c i o - f l u v i a l accumulations. Below the Donjek these are suddenly replaced by high abrupt rock c l i f f s * and the va l l ey instead of having an o ld appearance, exhibits considerable evidence of being a youthful depression. Thus in a l l probabi l i ty a form-er r i ve r drained through the present va l ley o f the White to the mouth of the Donjek, but thence fol lowed some other route to the pre-Glacial channel of theYukon r i v e r . The Upper White, according to this system of drainage, came very close to the Generc and, due possibly to g l a c i a l accumulations, was eventually turned into i t over the present posit ion o f the Upper canyon. ( 1 ) Cockfield i s of the opinion that from where the Pe l ly r i v e r swings sharply to the south to jo in the Yukon r i ve r i t has l e f t i t s old v a l l e y . This ancient va l ley i s approximately ten miles wide and extends to jo in the Stewart r i v e r . There i s a d i f f e rence o f e leva-t ion in the ancient va l ley of about 300 f e e t , t l ) Cockfield W.E. Personal Communication 1930. (33) He also stated that the Yukon r i v e r enters a t o t a l l y d i f f e r en t va l l ey north o f i t s junction with the Pe l ly r i ver * Sooth o f this junction the Yukon r i v e r f lows in a wide, f l a t va l l ey with rounded h i l l s while north o f this junction i t f lows in a narrow steep-walled v a l l e y . He attr ibutes these changes to accumu-la t i on o f g l a c i a l debris* Farther de ta i l ed invest igat ion w i l l be necess-ary before the drainage changes w i l l be understood, and the former drainage system i s established* (54) GENERAL GEOLOGY* & great var ie ty of rocks, both sedimentary and igneous and ranging in age from Pre-Cambrian to Secant, are found throughout the Yukon t e r r i t o r y . The oldest rocks are metamorphic and are domin-(1 ) antly schistose in character. They are considered to be, mainly, o f sedimentary o r i g in . These metamorphic schistose rocks consist dominantly of schistose amphi-bo l i t e s , quartzlte schists, mica schists,, and occasion-a l beds o f limestone and are known as the Yukon group. Occasional dykes and small intrusive bodies pierce the Palaeozoic beds in places, and greenstones of various types have a considerable developement in association with certain of the Lower Cambrian or Pre-Cambrian members, and occur as s i l l s , dykes or larger irregular intrusive masses. The evidence obtained regarding these rocks, indicates rather conclusively that they are a l l of Pre-Cambrian age. Overlying the metamorphic schistose rocks i s a group of rocks composed mainly of dolomites, quart-z l t e s shales, s la tes , phy l l l t e s , and associated green-stones. These are known as the T indir group and are considered as younger than the Yukon group of Pre-Cambrian age. (1 ) Cairnes D.D. "The Yukon-Alaska International boundary between Porcupine and Yukon r i v e r s . " G.S.C. Mem 67 P. 35 1914. (35) The Tindir group i s over la in, unconformably, by a thick ser ies of limestone-dolomite beds which range in age from Cambrian to Carboniferous. Shales, cherts, sandstones, cherty conglomerates and thinly bedded limestones are found ranging in age from Ordo-v ic ian to Carboniferous. These beds are in turn over-la in by a thick ser ies of sediments composed domlnant-l y of shales, sandstones and conglomerates with occas-ional intercalated beds of limestones that are of Pennsylvanian or Permian age. These compose the Nation River formation. Apparently conformably, overlying the Nation River formation i s a thick series of Hesozoic sediments including mainly shales, sandstones, greywackes, con-glomerates, s lates and quartz i tes. Fossi ls have been found in these rocks indicating a Cretaceous age. Invest igat ions, so f a r , have f a i l e d to show any large basins of Tert iary sedimentation as occur in the In t e r i o r plateau region of Br i t ish Columbia. A few small basins of sedimentary Tert iary are known to occur in the central portion of the Yukon plateau. More recent than these consolidated roek formations are the super f i c ia l deposits o f s i l t s and g l ac i a l debris of Recent and Pleistocene times. These Recent and P le is to-cene deposits form a mantle obscuring the older forma-tions throughout a large portion of Yukon. (36) The igneous roeks throughout the region consist o f intrusive and extrusive phases. The intrusive rocks are represented by the Coast Range intrusives, which consist mainly of grano-dior i t ic roeks. They form the Coast Range bathol i th and occur as outlying stocks in the plateau region. These rocks are considered of Jurassic or younger in age. Older than the Coast Range intrusives i s a ser ies o f igneous rocks composed o f d i o r i t e s , andesites, andesit lc t u f f s , pyroxenites, amphibolites, and magne-s i t e s . Cf the i r age nothing i s known except that they are older than the Coast Range. The extrusive phase of the igneous rocks i s represented by a series of f lows, t u f f s and breccia. Younger than the Coast Range intrusives i s a wide-spread ser ies o f andesites and andesitic t u f f s and breccias. Some of these are considered contemporaneous with and some younger than the Jura-Cretaceous sediments. More recent than these andesitic f lows are la te Tert iary or Pleistocene basalts occurring mainly in the ibrm of f lows* These l a t e r flows were, in places, accompanied by extensive accumulations o f basalt t u f f s . About this time a number o f dykes and small stocks of granite and syenite-porphyry were also intruded, but i t i s not known whether before or a f t e r the basalts. The most recently (37) consolidated recks consist of a series of rbyolites, trachytes, and lat i tes , which pierced the older rocks and poured over the country in places, generally in sheets 50 feet or less in thickness. These outpourings were accompanied by great quantities of tuf fs and breccias. Scanned by UBC Library The oldest pocks known to occur in Yukon are the Yukon group, and consist mainly o f mica schists, quartz-mica schists, and schistose amphlbolites, which are considered to be o f Pre-Cambrian age* These rocks are so highly metamorphosed that they a f f o rd very l i t t l e information concerning this early period in the geolog-i c a l h istory of the area. They show that great thick-ness o f arenaceous and argi l laceous sedinents were de-posited in a probable Pre-Cambrian sea which occupied th is region at this time. They also show that these sediments have been invaded by various igneous rocks, and that a l l have since become so metamorphosed as to be dominantly schistose or gneissoid in character. The Yukon group probably represents the north-ward extension of the Pre-Cambrian rocks o f central Br i t ish Columbia and with the T indir group of r e l a t i v e l y unaltered sedinents may possibly represent a continua-t ion o f the Pre-Cambrian sea-way from southern Br i t ish ( 1 ) Columbia to the Arct ic ocean as suggested by Schof ie ld. The Tindir formation may also comply with Schof ie ld 's Belt ian o f the Cranbrook region of Br i t ish Columbia. During Lower Palaeozoic times great thickness-( g j Schof ie ld S.J. Trans. Roy. Soc. of Can. XVI I I . P t . A. 1925. (39) es o f argi l laceous and arenaceous matter, fol lowed by calcareous materials, were deposited in a sea-way. Vulcanism was also ac t i ve . The Lower Palaeozoic term-inated in Yukon with a wide-spread, dynamic revolution which caused extensive deformation and metamorphism, and was accompanied by considerable volcanic a c t i v i t y . I t i s probable that some of the intrusivos may have been Injected at this time. Where the records are s t i l l l e g i b l e , they show that at the close o f the Si lurian disturbance a consider-able area was above the sea and a long erosion in terva l ensued. Some time before the Middle Devonian a great part of Yukon sank beneath the sea, and at about that time vulcanism became active at a number of points. This sea-invasion, which commenced in Devonian time, continued wel l into the Carboniferous period. The normal course of sedimentation was, however, repeatedly interrupted by volcanic a c t i v i t y , as a result of which andesitlc and basalt ic lavas were extruded. I t seems probable that certain members of these volcanics may be ( 1 ) contemporaneous with the Carboniferous sediments. Marine occupation with i t s accompanying sedi-mentation continued from Carboniferous unt i l wel l into Cretaceous time, and during the Mesozoic epoch, arena-(1) Cairnes D.D* "Upper White River d i s t r i c t , Yukon," G.S.C. Mem 50 P.112 1915. (40) ceous and argi l laceous sediments were deposited which have now become al tered into shales, a r g i l l i t e s , sand-stone, greywackes, and conglomerates* Throughout the western portions o f Br i t ish Columbia and southern Yukon, a widespread crastal move-ment occurred in Jurassic times and possibly e a r l i e r , t o l a t e r , which was accompanied by the intrusion of vast amounts o f igneous materials including a great part, at l e a s t , o f the roeks composing the Coast Range bathol i th. At the close o f the Jurassic disturbance a considerable area was above the sea, and what was probably a short period o f erosion ensued* Cretaceous roeks are known to contain pebbles of g ran i t i c rocks, which are s imi lar , l i t h e l o g i c a l l y , to the Coast Range Intruaives, and to be eat by l i t ho l o g i e -a l l y s imilar g ran i t i c rocks. I t i s evident, there fore , that gran i t i c intrusions apparently derived from the same magma, have invaded this same general be l t at d i f f -erent and, in some places, widely separated periods ranging from sometime in , or previous t o , the Jurassic un t i l at l eas t the close of the Cretaceous. The Mesozoic era, and part icular ly the l a t t e r part o f i t , was also characterized by volcanic ac t i v i t y as a result o f which the Mesozoic and Carboniferous sediments became intensive ly invaded by andesites, dia-bases, basalts , and re lated roeks. As mentioned pre-(41) viously these rocks were considered to be interbedded with the Carboniferous sediments, and i t was thus con-H i sidered possible that these e lder volcanics represent a long intermittent period o f vulcanism extending from (2) Carboniferous unt i l l a t e Cretaceous times. Coekf ie ld, however, I s o f the opinion that these Carboniferous rocks belong high up in the Mesozoic era. The Mesozoic period o f sedimentation was term-inated, at or about the close o f Cretaceous time;by a widespread deformation, at the close of which a consider-able portion of Yukon was above the sea. Degradation became ac t i ve , and no evidence has, so f a r , been obtain-ed to show that from then to present time, any portion of this region has been subjected to marine conditions. During early Tert iary time fresh-water sedim-ents were deposited throughout considerable portions of Yukon. The beds were f o r the greater part at l eas t , deposited in shallow basins and in places contain seams of l i g n i t e . Previous to the deposition of the early Tert iary beds, and a f t e r the deposition of the Cretaceous sediments, an extensive and far-reaching period of deform-ation ensued corresponding apparently to the Larrimide revolution elsewhere. ( l )Ca imes D. D. "Uuper White River d i s t r i c t Yukon." *" G.S.C. Mem 50* P. 114 1915. (g )Cockf le ld W.B* Personal Communication. 1930. (42) Af t e r the deposition o f the Tert iary beds, a gradual u p l i f t occurred which though of orographic character, was in places accompanied by volcanic act iv -i t y and by a considerable disturbance o f the Eocene sed-iments. This movement a f f ec ted apparently a l l of the Yukon plateau as wel l as the Coast range. ( 1 ) Much discussion has ensued about the actual time of this period of crustal s t ab i l i t y but i t would ( 2 ) seem from evidence given by Caimes that the planation o f the Plateau region was contemporaneous with the de-posi t ion o f the Eocene beds. Some discussion has ensued both f o r and against the synchronous planation o f the Yukon plateau and the Coast range by the above wr i ters . (2) Caimes i s o f the opinion that they were synchronously eroded but be l ieves that considerable r e l i e f remained. ( 2 ) Caimes bel ieves the Coast range and the Yukon plateau were synchronously up l i f t ed but that the Coast HI) Brooks A.H. "The geography and geology of Alaska,^ U.S.G.S. Prof.Paper #45 PP 292,293 1906. Spencer A .C. "pac i f i c Mountain systems in Br i t ish Columbia and Alaska;" Bull.Geol.Soc.Amer. Vo l . 14 1903. PP. 117-132. Dawson G.M+ "On the l a t e r physiographlcal geology of the Rocky Mountain region in Canada, with special references to changes in e levat ion and the history of the g l ac ia l period. Trans.Roy.Soc.Can.Yol.111 Sec IV 1890 PP. 11-17. Spurs J.E. "Geology of the Yukon Gold d i s t r i c t , Alaska." U.S.G.S. 18th An.Rp. P t . I I I . . 1898 PP 260,262,263. (2 ) Caimes D.D."Upper White River d i s t r i c t , Yukon," G.S.C. Mem 50.PP. 115-118 1915 (43) range being near the edge was up l i f t ed more than the Central t ract which was near the axes. The Tert iary period, part icular ly a f t e r Eocene time, was also one of persistent volcanic a c t i v i t y . I t i s probable that these f l a t - l y i n g lavas were poured over (1 ) the surface from vents and are considered to have flowed out over a region of considerable topographic d i ve rs i t y . These f lows are l a t e r than the dissect ion that fol lowed (2) the u p l i f t of the plains and are indeed Recent. Later than the above lavas and in pre-Glacial time voleanic ac t i v i t y was present in Yukon. These voleanlcs occur both as intrusives and extrusives and apparently a l l or ig inated from loca l vents. ( 1 ) Caimes D+D.*Upper White River d i s t r i c t , Yukon." G.S.C. Mem. 50 P.118 1915. (2 ) Mendenhall W.C. "Geology of Central River region Alaska." U.S.G.S. Prof paper #41, 1905, P. 57. o— ^ ^ t ^ ^ § 53 ^  - 4 F -O ^ ^ ^ A S t - t ^ -^ ^ ^ ^ ^ ìi ! s í s ? ^ S --o 5 - o j x: -oo $ o Si h ^ N ì M i ! .Ss st S! { S (44) PRE-CAMBRIAN ROCKS. The Pre-^ambrian rocks of the Yukon region are grouped, in this thes is , under the term Yukon group. They are exposed over a large area of the Plateau region and are considered to be possibly underlying the whole of the Yukon plateau. The rocks of th is group are dominantly schis-tose in structure and consist mainly o f quartzite schist schistose amphibolites ch lor i te schist , granite-gneiss, and mica schists but also Include thin bands of crysta l -l ine limestone* A l l these members are much fo lded , faulted and d is tor ted , and are so metamorphosed that in places i t i s d i f f i c u l t to or impossible to determine ( 1 ) the i r o r i g in or o r i g ina l characters* The quartz i te-schists are character is t i ca l ly l i gh t to dark green, f i n e l y textured rocks which have a decidedly schistose structure, but which cleave imper-f e c t l y along the planes of sch ls tos i ty , and break pre-va i l i ng l y into rough, somewhat platy or occasionally into prismatic fragments, due to more than one set of cleavage surfaces being developed. -Under the microscope these rocks are seen to consist dominantly of intergrown (1 ) Cairnes D.D. "Yukon-Alaska International Boundary between Porcupine and Yukon r i v e r s . " G.S.C. Mem 67. P. 38 1914. (52) and inter f ingered quartz grains associated with which are varying amounts of ch lo r i t e , ca l c i t e and iron ore . Genetical ly these rocks are sheared and metamorphosed quartzitea and related sediments. The amphibolites are character is t i ca l l y f i n e l y -textured, dark green rocks with a marked schistose structure, which cleave only imperfect ly , however, along the planes of schistosi ty and break as a rule into rough, i r regu lar , o f t en somewhat prism shaped fragments. When examined under the microscope these roeks are found to be composed mainly of green hornblende, dlopside, and carbonates, but contain, a lso , varying amounts of quartz, f e ldspar , s e r l c l t e , sphene, and iron ore* These amphi-bo l i t e s are evidently impure sandstones, greywackes,or arkoses, that have become much a l te red . The mica schists are grey, medium-grained, and with a pronounced developement o f mica on the planes of sch is tos i ty . They consist of quartz and mica, with subordinate fe ldspar and ch lor i te* The proportion of quartz to mica var ies widely in d i f f e r en t specimens; ranging from types in which mica i s abundantly developed to others in which the mica i s rather sparse, and which approach quartzites in composition. The quartz-grains are usually intergrown. The mica i s in para l l e l bands that occasionally show intense p l i ca t i on , even in a band specimen, sweeping in a series o f curves through (46) the specimen. The c l o r i t e schists are bright green to grey rocks with pronounced f o l i a t i o n , and a g l is tening appear-ance on a f reshly broken surface. They consist of vary-ing amounts of ch lo r i t e , b i o t i t e , and hornblende, with laths o f fe ldspar , and minor amounts of magnetite. The granite-gneiss i s grey to pink, with char-ac t e r i s t i c gneissoid texture, ;nd in some l o c a l i t i e s with an abundant developement of crysta ls of fe ldspar forming an augen gneiss. I t consists essent ia l ly o f quartz, orthoclase, p lag ioc lase , b i o t i t e or hornblende or both, and micropegmatite. The quartz and feldspar In some cases show granulation, but in most cases the quartz grains are intergrown, with a satured texture. Mica when present i s arranged in para l l e l bands. In some specimens the individual leaves of mica show bend-ing or crushing against an individual of quartz or f e l d -spar. The limestones vary from white to brown, and are more or less impure, being usually quite s i l i ceous . P rac t i ca l l y a l l traces of the o r i g in ;1 bedding has been obscured. These c rys ta l l ine limestones appear in places to represent Info lded portions of more recent beds but in other places, however, the limestone i s intercalated somewhat regular ly , and has every appearance of being, (47) and probably i s contemporaneous with the schistose (1) members. The age and corre lat ion of these metamorphic schistose rocks had af forded geo log is ts , working in (2) Yukon, a great deal o f trouble. Cairnes apparently has the i r horizon de f i n i t e l y determined. He found Pro-Middle Cambrian sediments to be over ly ing the schistose rocks. The next d i f f i c u l t y i s the corre lat ion o f pro-bably s imi lar rocks of other areas with th is known area. The other exposures o f metamorphic rocks are correlated by l i tho logy which i s a rather uncertain method. In many places there s t i l l remains a great amount of un-cer ta in l ty regarding the i r age and o r i g in . S t i l l I t seems f a i r l y certain that there does ex is t a Pre-Cam-brian ( ? ) metamorphic complex underlying a l l the sed-(3) imentary rocks of known age throughout Yukon. (1 ) Cairnes D.D. "Yukon-Alaska International Boundary between Porcupine and Yukon r i v e r s . " G.S.C. Mem.67. P.39 1914. PP. 40-44. "Yukon-Alaska boundary between Porcu-pine and Yukon r i v e r s . " G.S.C. Mem 67.P.44. (2) (3 ) Idem Cairnes D.D. (48) PALAEOZOIC ROCKS. CAMBRIAN Pre-Middle Cambrian. ( 1 ) Books of this age have been described as being composed dominantly of sedimentary rocks, but includes a lso, in most places, some basic volcanics which are In a general way designated as greenstones. The sedimen-tary rocks include, mainly, quartz i tes , dolomites, shales, s lates phy l l i t e s , and also occasional beds of conglomerate and magnesite. In d i f f e r en t l o c a l i t i e s the formation var ies great ly in i t s general l i tho l og i ca l aspect due to the predominance of certain members. These rocks are a l l grouped together as Pre-Middle Cambrian in age, however, since these beds are unfoss i l l f e rous the grouping and corre lat ion depends upon l i t ho l o g i c a l character is t ics , which are by no means i n f a l l i b l e . These Pre-Middle Cambrian rocks have been des-cribed from only one l o c a l i t y , namely, along the 141st. meridian between Porcupine and Yukon r i v e r s . The quartzite are dominantly white to l i gh t grey in colour, g iv ing them a resemblance to limestone. They are almost universal ly f i n e l y textured and thinly bedded. Microscopically these rocks consist dominantly of inter locking and intergrown quartz and fe ldspar (49) grains, with which is always associated a certain amount of s e r i c i t e and carbonate, that occurs as a binder or matrix f i l l i n g the comparatively s l i ght amount of inter-s t i t i a l space throughout the rock mass. The dolomites much resemble the f ine-grained, greyish quartzites in appearance and are similar to them in hardness. They are dominantly l i ght gray to yel low-ish in colour, and are nearly everywhere thinly bedded. The dolomites also contain intercolated seams of chert and quartz i tes . The shales include greyish to black, thinly bedded, f l aky , non-calcoreous members, as wel l as other l ess thinly bedded black, so f t shales which readily decompose to form black mud. Although the rocks are highly fo lded and f au l t -ed, metamorphism i s not pronounced in the d i f f e r en t members of the group; they have nowhere a schistose or gneissoid structure and seldom possess a s laty cleavage. They are thus very d i f f e r en t in this respect to the Yukon group. Occasional dykes and small intrusive masses of diabase pierce these rocks, in places, and since the dykes rarely extend up into the overlying Palaeozoic rocks, the diabase i s probably also o f Pre-Middle Cambrian age. (50) No f o s s i l s were found in this group of rocks, but the beds underlie Devono-Cambrian limestones in which Middle Cambrian f o s s i l s were found. Below this Middle Cambrian horizon several hundred f e e t of l i tho -l o g i c a l l y s imi lar , but unfoss i l l f e rous limestones and dolomites, which in a l l probabi l i ty represent the Lower Cambrian, occur and underlying these beds unconformable* ( 1 ) occur the Tindir rocks described above. Caimes i s of the opinion that the Tindir rocks are ent i re ly Pre-Cambrian or that this group includes both Lower Cambrian and Pre-Cambrian members. I t i s considered that these rocks may belong (2) to the Belt Terrane of Br i t ish Columbia. I t also appear that the Pre-Cambrian i s extensively developed in por-t ions of Yukon, and that these rocks are d i v i s i b l e into an upper but s l i gh t l y metamorphosed d iv i s ion , the Tindir rocks, and a lower highly metamorphosed d iv i s ion , the "Yukon group. Cambrian - S i lur ian. (3 ) Rocks of this age are described by Caimes, and are only known, so f a r , to occur in th is one l oca l -i t y . He f inds that these rocks are restr ic ted to the (1 ) Catrnes D.D. * Op. Citl" P7"56l * (2 ) Daly R.A. G.S.C. Mem. #38, PP.179-191 1912. Schof ie ld S.J. "Reconnaissance in East Kootenay,B.C. G.S.C. Sam.Rp. 1912 PP. 221-225. (3 ) Caimes D.D. "Yukon-Alaska International Boundary between Porcupine and Yukon Rivers, " G.S.C. Mem 67. P.58. (51) higher mountainous tracts of Keele and Ogi lv ie moun-ta ins . The rocks consist of white to l i ght grey lime-stones and dolomites. They are dominantly crys ta l l ine and in places beds of part icular ly good marble occur. In texture these limestone-dolomite rocks vary from f i rm dense, dolomites to coarsely c rys ta l l ine almost pure limestones. They also appear somewhat massive. The dolomites are considered to have been derived from the limestone. In a few places, greyish, yel lowish, to nearly black shales are intercalated with these limestone-dolomite beds. They are, however, of minor importance. Cairnes found i t prac t i ca l l y impossible and Impracticable to separate the Si lur ian, Ordoviclan and Cambrian beds. They are a l l much folded and fau l ted . These limestone-dolomite rocks are over la in by Devonian limestones, ^hich a r ; , in turn, over la in by Carboniferous limestones, cherts and related rocks. The f o s s i l s obtained from the limestones gave agee from Cambrian to Si lur ian, and the Cumbrian, Ordovician, and Si lurian horizons were recognizable. Devonian. Limestones of this age have been described by ( 1 ) Cairnes from the Keele and Cg i l v i e mountains where they are intimately associated with the Cambro-Silurlan group (52) of limestones and dolomites. Shale-chert rocks ranging in age from Ordovi-( 1 ) elan to Devonian have been described by Caimes and Cockfield from many l o c a l i t i e s in Yukon region. The limestone beds resemble very c losely those of the Cambro-Silurian and f o s s i l s , alone, serve as the distinguishing feature. They are more homogenous and darker in appearance, the colour being t yp i ca l l y dark, bluish grey. They are also somewhat c r ys ta l l i ne . Their age i s d e f i n i t e l y f i xed by f o s s i l remains and the rocks have been correlated l i t h o l o g l c a l l y with known areas of Devonian limestone in Alaska. The shale-chert ser ies consists dominantly of shales and cherts which are preva i l ing ly c lose ly and f i n e l y interbedded. They vary from cherty shales to shaly cherts. The cherts are general ly dark grey to black in colour &nd the shales are grey to black or bluish black in colour, the darker beds being decidedly calcareous in character. Devonian f o s s i l s have been found in these shale-chert beds and they d i r ec t l y ove r l i e middle or lower Cambrian limestone beds. They are, in turn, over la in by Carboniferous shales. Shale-chert beds, probably s imi lar to these beds, have been described from various (1 ) Caimes D.D. Op*Cit. P. 83 (53) (1 ) places in Yukon but Caimes a f fords the most accurate data, to date, as regards the i r age. They have been, by Caimes, re ferred to the Cache Creek ser ies o f Br i t i sh Columbia. Carboniferous. Rocks ascribed to the Carboniferous period have an extensive developement in Yukon. However, there has been considerable d i f f i c u l t y in determining the i r age. ( I ) Cairnes a f fords us the f i r s t c lear intimation of the i r age, in his work along the 141st. meridian. Here he describes two geo log ica l formations, one the shale group, and the other the limestone chert group. The shale group i s , apparently? only known in the v i c -i n i t y o f the 141st. meridian. This shale group consists dominantly of shales but includes also c lays, cherts, calcareous sandstones, and thinly bedded limestones. The whole group Is domin antly dark in colour. Since these shale beds rest upon Devonian shales and cherts i t would seem quite possible that Rlssissippian members are also included in the shale group, unfortunately f o s s i l remains were not found. ( 1 ) Cairnes D.D, Op. C i t . P. 83 (54) The limestone-chert group consists mainly o f limestone and chert , but includes also occasional beds of dark shale, calcareous sandstone and cherty conglom-erate . The limestone beds are quite c rys ta l l ine and range from white to grey to almost black in colour. These beds contain both Pennsylvanian and Miss-iss ip lan f o s s i l s . They ove r l i e the Devonian limestones and are over - la in by Pennsylvanian beds. Another group of rocks the Nation River forma-t i on , was described by Caimes from this same area. These consist dominantly o f conglomerates, sandstones, and shales but include also occasional beds of limestone. The pebbles of the conglomerate consist dominantly of chert and are small (1 inch) in s i z e . The sandstones are character i s t i ca l l y greyish to brownish, medium tex-tured, hard, f irm rocks. The shales are dominantly greyish to yel lowish in colour and range in character from f r i ab l e to hard and somewhat s la ty , and from quite f ine to coarse and arenaceous* The limestones are l i gh t grey, semi-crystal l ine to c rys ta l l ine beds and occur general ly as thin beds intercalated with the more arena-ceous and argi l laceous sediments. Imperfect plant remains were found, thus plac-ing the beds as of possible Carboniferous age. A conglomerate consisting of a f i rm, somewhat dense, f i n e l y textured, reddish, argi l laceous matrix, (55) in which are embedded angular to sub-angular pebbles and boulders ranging in s ize from microscopic to 3 or 4 f e e t in diameter. The matrix appears to have approx-imately the composition o f a boulder c lay , and the great-er number o f the pebbles and boulders are composed of limestone or dolomite, but some were noted composed of ether sediments such as sandstone, conglomerate, and shale. The prevai l ing red colour of the matrix i s due mainly at l eas t to the considerable amount of iron contained in the matrix which has in places the appear-ance of hematite o re . The conglomerate i s quite un-s t r a t l f i e d and has the general appearance o f a consol i -dated and iron-stained boulder c lay . The pebbles and boulders are i r regu lar ly distr ibuted and are o f t en quite i so la ted and completely surrounded by the matrix, instead o f rest ing upon one another as in the case o f a normal conglomerate. This conglomerate i s undoubtedly o f t e r r e s t r i a l o r i g i n . In general composition,this conglomerate, re-sembles boulder c lay more than i t does s l ide mater ia l , but on the other hand, i t s prevai l ing reddish colour, and the fac t that this conglomerate has not been des-cribed from other portions of Alaska or Yukon, and i s thus probably net very extensive would tend to disprove the g l a c i a l theory o f o r i g i n . Although s t r ia ted pebbles (56) (1) were not found Cairnes thinks they might occur. Pebbles having facetted surfaces, much resembling "so led" pebbles were noted to be somewhat p l e n t i f u l , The con-glomerate i s considered to be too extensive f o r s l ide mater ia l . This taken in conjunction with the thickness of the conglomerate Cairnes thinks rather favours the g l a c i a l theory. This conglomerate ove r - l i e s the Devono-Cambrian limestone-dolomite beds and appears to ove r l i e Carbonif-erous shales and Devono-Crdovician shale-chert group and to s t rat lgraphica l ly correspond to the Nation River formation. The conglomerate was thus probably formed about Carboniferous time, and may correspond to the Permo-Carboniferous t i l l i t e s or conglomerates considered to be o f g l a c i a l o r i g in , that occur in South A f r i c a , Austral ia , India and other parts of the world. Along the boundary l i ne Cairnes found three f a i r l y d e f i n i t e l y i d en t i f i ab l e Carboniferous horizons. The lowest i s represented by Misslssipian f o s s i l s from the lower portion of the limestone-chert group. The next more recent horizon i s represented by Pennsylvanian f o s s i l s obtained from the shale group and the upper por-t ion of the limestone-chert group. The most recent horizon i s represented by f o s s i l s from the Nation River (1 ) Cairnes D.D. "Yukon-Alaska Boundary"between Porcupine and Yukon r i v e r s , " G.S.C. Mem 67.P.92, 1914. (57) formation which is o f Upper Pennsylvanian or possibly Permian age. Limestones, cherts, and related rocks that l i t ho l o g i ca l l y resemble the members of the llmestone-( 1 ) ( 2 ) chert group are described by McConnell. Caimes i s of the opinion that the members of this group are also pro-bably included in the Braebum limestones which are ex- -(3 ) tensively developed in Yukon and northern Bri t ish (4 ) Columbia and are dominantly of Carboniferous age, but . (5 ) may include also Devonisn members. Cockfield, a f t e r the f i e l d season of 1929, i s of the opinion that he has su f f i c i en t f o s s i l evidence, which was hitherto lacking, to place the Braebum limestones high up in the Mesozoic succession. The writer f e e l s that the Braebum limestones should be described here rather than be placed in an inde f in i t e period of the Mesozoic era. These limestones are generally sub-crystal l ine, in places approaching marble, but in others quite f laggy and argil laceous or ( 1 ) McConnell R .d . An.Rp.G.S.C. Vol.XV,19u2, 31A-34A. (2 ) Cairnes D.D. "Yukon-Alaska Boundary," G.S.C. Mem. 67 F. 103. 1914. (3 ) Caimes D.D. "Preliminary memoir on Lewes and Norden-skiold Rivers coal d i s t r i c t , " G.S.C. Mem 5. 1910 PP.28,29. (4 ) Cairnes D.D. "At l in d i s t r i c t , Br i t ish Columbia." G.S.C. Mem 37 1912, PP. 53-54. (5) Cockfield W.E. Personal Communication 1930. (58) even s i l i ceous . They are commonly white to l i gh t bluish (1) in colour. Cairnes states that " th is limestone appears to be the same as that of the Upper Cache Creek ser i es , found to the south in the Conrad Mining d i s t r i c t , and therefore of Carboniferous age. The evidence af forded by the f o s s i l s co l lected has been of ¿.n inde f in i t e character, and the inference that the t^o formations are of the same age has bean mat - mainly on H tho l o g i c a l grounds." As previously stated Cockfield f e e l s certain that recent f o s s i l evidence - i l l place these limestones in the upper Mesozoic era. I f this happens i t *.vill help great ly in unravell ing the volcanic complex. Limestones similar to the Braeburn limestones have been described by Cockfield from the Whltehorse Dis t r i c t and as regards the i r age he states:-"The evidence as to the an;o of the limestones i s very unsat is factory. I t i s how-aver, quite possible that l inastones belonging to more than one geolog ic period have been included. The most de f in i t e evidence to date i s the f inding of the genus fus i l i na by 9.M. Dawson, vhich would demonstrate that part at least of the limestone i s Carboniferous. Some evidence has been presented to show that Devonian and Tr iass ic faunas may also be represented, h-r-.t as yet IT.)" Cockfield W.E. and Bol l AjlH "Whitehorse d i s t r i c t , Yukon." G.S.S. Mem. 150 P.14 1926. (59) no progress has been made in subdividing the limestone into d i f f e r e n t formations. The evidence as a whole i s too meagre to permit of stating that Devonian and Tr lass ie are represented." (1 ) Cockfield describes cherty quartz i tes , black s la tes , b i o t i t e s lates and limestone from Windy Arm and Tagish lake . These beds extend toward At l in and are fo l lowed by general ly c rys ta l l ine limestone, from which fragments of f o s s i l s are obtained. Without further statement he r e f e r s the limestone to the Upper Carbonif-erous or Tr iass ic thus placing the quartz i tes , s lates and the other band of limestone as.Pre-Upper Carbonifer-ous. Rocks ranging in age from Ordoviciah to Car on-(2) i f erous have been described from the Dezadeash Lake area. They consist of dark, coarsely-bedded a r g i l l i t e s seldom strongly cleaved, but in some instances consider-able mica has developed and they pass into a sch is t . These rocks also contain greyish quartz i te and some con-glomerate but no limestone was found. They dip at steep o angles from 50-85. No f o s s i l s were found. The only evidence as to the i r age i s that they ove r l i e Pre-Cam-brian ( ? ) schists and are cut by g ran i t i c in t rus ives . (1 ) Cockfield Explorat ions in 6. Yukon," 6.S.6.**** Sum. Rp. 1922. (2 ) Cockfield *.?.E. "Dezadeash Lake Area, Yukon," G.S.C. SHm. Rp. 1927. (60) Since the age o f the g ran i t i c intrusives i s assumed as Jurassic, Cockf ie ld places these sediments as ranging from Crdoviclan to Carboniferous in age. UBC Scanned by UBC Library (61) MESOZOIC ROCKS. The most wide-spread sedimentary horizon of the Meaozoic era i s represented by sediments ranging in age from Jurassic to Cretaceous. This horizon appears to be more wide spread in Southern Yukon than in the northern port ion. Igneous rocks of the Mesozoic era are very ex-tensive ly wide-spread throughout Yukon and w i l l be des-cribed in some de ta i l under the heading "igneous Rocks." T r iass i c . Limestone occurring in patches and bands in ( 1 ) andesit ic rocks has been described by Cockf ie ld . These bands are seldom more than 5C0 f e e t wide and the bedd-ing i s i n most places obscure, and d i f f i c u l t or imposs-i b l e to detect . The limestone i s grey and compact, and l a r ge l y , i f not, wholly r ec rya ta l l ine . From the poor f o s s i l s co l lected there would appear to he two bands of l imestone, one of Carboniferous ( ? ) age and the other of Tr iass ic ( ? ) age. Phase cannot be separated but i t i s probable that Mesozoic limestone may be present. Cockfield also describes a ser ies of conglom-erates and a r g i l l i t e s from this same area. The conglom-erates are massively bedded with cobblas and boulders of (1 ) Cockfield W.K. " L i t t l e Salmon niv^r area, Yukon," 3.S.C. Sum. Rp. 1928. (62) grani te , andesite and schistose rocks in a tuffaceous matrix. A few exposures of dark grey to black a r g i l l -i t e s were found. The a r g i l l i t e s have been jointed and sheared so that the bedding i s indist inguishable. Fossi ls obtained from the a r g i l l i t e s correlated the beds as belonging to probable Upper T r i ass i c . The Laberge beds have been determined in White-horse d i s t r i c t as o f probable Lower Jurassic age to Middle Jurassic age. I t seems apparent that a r g i l l i t e s above do not -belong to the Laberge beds, but constitute an underlying formation. "Occasional f o s s i l s that were bel ieved to be Tr iass ic forms have been heretofore found in southern Yukon, in a l l cases in limestone, but the i r poor state of preservation prevented a d e f i n i t e age determination. I t would now appear to be f a i r l y d e f i n i t e l y established that marine Tr iass ic i s much more widespread in southern Yukon than was formerly bel ieved to be the case. I t i s also possible that some marine Tr iass ic has been included with the Laberge beds in other sect ions. Jurassic. Sedimentary rocks have been assigned to the ( 1 ) Jurassic period by Cockfield in the Whitehorse D i s t r i c t . Here Cockfield f inds these rooks extensively developed (1 ) Cockfield Y'.E. and Be l l A.B. "Whltohorse d i s t r i c t , " Yukon," G.S.C. Mem. 150 p.15 1926. (63) in the eastern ha^f of the d i s t r i c t . They occur as long, somewhat narrow be l ts with a trend a l i t t l e east of north and south of west, pa ra l l e l to the Coast range, (i) Caimes has determined the i r thickness as between 5,000 and 6,000 f e e t . These rocks consist domlnantly of v o l -canic c l as t i c s , t u f f s , breccias, and conglomerates. Interbedded with the t u f f s are considerable thicknesses of marine shales and a r g i l l i t e s in which tufiaceous material i s an important constituent. In a few places thin beds o f .dsrk, impure limestone are interbedded with the bedded t u f f s . This group of rocks Cockfleld has ca l led the Laberge series and includes in the ser ies the Tantalus conglomerate. Caimes, In his repcrt on the Conrad and Whitehorse mining d i s t r i c t s , groups these two formations under the name Tutshi se r i es . (2) In Wheaton d i s t r i c t where the best exposures of t yp i ca l l y marine sediments occur, a three f o l d d i v i s -ion of the Laberge beds was recognized and i f the Tanta-lus conglomerate be included, a f our - f o ld d iv i s ion was determined. The lower beds of the Laberge ser ies con-s is t o f arkoses, t u f f s with shales and conglomerates. The middle members are composed o f shales, sandstones and arkoses; and the upper beds consist ch i e f l y of (1 ) Caimes D.3. "Wheaton d i s t r i c t , Yukon," G.S.C. Mem 21, 1912. ( 2 ) Caimes D.D. Op. C i t . (64) sandstone* Overlying those div is ions i s the Tantalus conglomerate which i s com osed of conglomerate, shale, sandstone, and coa l . Caimes notes that these d iv is ions are only approximate and that the thickness of each var i es . In the lower beds the arkoses are general ly l i gh t to dark grey or pale greenish, but occasional red beds are found. They have a dense texture and are f i rm, compact rocks, which occur in heavy, massive beds, so that the s t ra t -i f i c a t i o n i s in many cases only distinguishable from a distance. Associated with the arkoses are t u f f s which so resemble them that i t i s general ly d i f f i c u l t to t e l l them apart. More than ha l f the lower beds of the laberge ser ies consists ch i e f l y o f these arkoses and t u f f s but the upper portion contain a considerable developement o f conglomerate and shale. The conglomerate occurs in thick, massive beds and consists o f material varying widely in s i z e . The pebbles and boulders range in s ize from that o f sand grains to 6-8 inches, o r more in d ia-meter and are mainly e i ther andesit ic fragments derived from the "Older Voleanics" or pebbles ident i ca l in com-posit ion with the Coast Range granodior i tes. The shales range from l i gh t grey to almost black, and usually form successive 20 or SO f e e t thiek and eacb o f a uni-form colour. (65) The middle beds consist chiefly of shales similar to those described above, but characteristically iron-stained and generally presenting a red appearance. When broken, however, they are seen to be grey to black, hard, dense rocks. These rocks occur in layers, with associated arkoses and sandstones, but the shales pre-dominate. The upper beds consist almost en t i r e l y o f medium-textured, somewhat f r i a b l e sandstones preva i l ing -l y greyish, ye l lowish, of l i gh t brown, and occur in heavy, nasslve beds. These d i f f e r g r e r t l y from the hard, dense, compact rocks noted in the middle beds. In the Wheaton d i s t r i c t the more truly c l a s t i c members of the group occur in a b e l t fo l lowing the mar-g in of the Coast range, and the members composed la rge l y o f volcanic material apparent ly increase in importance to the north and east . ( 1 ) In the Whltehorse d i s t r i c t the t u f f s of the Laberge ser ies are o f two main types; 1 . f ine-gra ined bedded and 2. medium-grained, massive. These two va r i -e t i e s occur intorbedded, but In places there i s a thick-ness of several hundred f e e t of one var i e ty without any admixture o f the other. The bedded t u f f occurs in some-what greater abundance than the non-bedded. (1 ) Cockfleld W.^. and Bel l A.H. "Whitehorse d i s t r i c t , Yukon," G.S+C. Mem 150 P.16 1926. (66) The non-bedded t u f f i s not eas i l y distinguished from an igneous reck sueh as diabase o r basa l t . I t i s an important member of the Laberge sc r i es . This rock has been described as a sandstone, arkose, or greywacke. I t I s f i n e to medium-grained and var ies in colour from l i gh t to dark grey and from grey to green. In some cases i t contains mica, and to the unaided eye appears to have the composition of a granite or d l o r i t e . I t i s interbedded with foss i l -bear ing shales or a r g i l l i t e s and with more or l ess f i n e l y laminated t u f f s * In the Whitehorse d i s t r i c t the Laberge ser ies appears to ove r l i e the Palaeozoic limestone with angular unconformity* The contact o f the t u f f s with the Coast Tange intruslves i s o f an extremely undulating nature and the fact that large apophyses are connected with the main body of the intrusive prove beyond doubt that the t u f f i s intruded by the granod5orite. Dykes o f g ran i t i c rocks, probably connected with the l a rge r in-trusive bodies, cut the bedded t u f f s and the greenstones with which the l a t t e r are associated. Foss i ls w^re found in the shale beds o f the Laberge ser ies In Whitehorse d i s t r i c t . Here i t has been ( 1 ) stated "that on the whole f o s s i l s ere of aomewhat rare occurrence." I T T ^ k i - i e l d and Be l l A.R, "Whitehorse d i s t r i c t , Yukon," G.S.C* Mem 150 P.21 1926. (67) The foss i l s from this district indicate the presence of strata ranging in age from Middle Lias to Inferior Oolite, that is o f strata of various stages, exclusive of the oldest, of the Lower Jurassic and earliest Middle Jurassic. Foss i ls have been co l lec ted from the Laberge bads in Wheaton, A t l ln , Whitehorse and Tantalus areas. In the co l lec t ions fron the Tantalus area, the specimens (1) were regarded as Jurassic or Cretaceous, but these species arc new regarded as Jurassic forms. Foss i ls co l lected in the A t l ln d i s t r i c t were reported on by Stanton as f o l l ows : "These may possibly be T r i ass i c , but I think i t more probable that they are early Jur-i s ) ass ic . They are cer ta in ly not as l a t e as the Cretaeeous." In the Wheaton and Whitehorse d i s t r i c t s Caimes reports numerous specimens which were regarded as possible young individuals of Prioncyclus woolgari . Cockf ie ld and Be l l are of the opinion that; " In view of the more pos i t ive evidence af forded by other co l lec t ions of f o s s i l s , i t seems highly probable that the tentat ive i d en t i f i c a t i on cf Prioncyclus woolgari by Whiteaves should be disregarded. Thus i t appears to be very we l l established that the Laberge beds range in age from (1 ) Caimes D.D+ "Lewes and Nordenskiold Rivers Coal District .* 6.S.C. Mem 5. PP. 34-35 (2 ) Gwil l in J.C. G.3.C. An.Rep. Vol X I I , Pt .3 , PP 23-27 189S. (3 ) Cockfield W.E. and Be l l A.H. "Whitehorse d i s t r i c t , Yukon." G.S.C. Mem 150 P.22 1926. (68) middle Lower Jurassic to lower Middle Jurassic. These rocks have formerly been correlated as of Cretaceous age. The Tantalus conglomerates have been included as 4 subdivision of the Laberge beds in this thesis. The conglomerates are rather wide spread in Southern Yukon and are of economic interest since they are hori-zon markers fo r the coal seams of Yukon. These conglomerates consist chiefly of massive beds of conglomerate, but also contain sandstones, shales, and coal seams. Their thickness as measured by Caimes in the Wheaton district is in the neighbour-hood of 1,800 feet . The conglomerates d i f f e r from a l l others of Yukon in that they are composed almost entire-ly of pebbles of quartz, chert and slate, the pebbles being generally cemented by a siliceous matrix. The component pebbles are remarkably uniform in size, rarely exceeding three inches in diameter and for the most part being between 1 and 2 inches in diameter. The associat-ed sandstone consists of thesame materials as the con-glomerates, but In a f iner state of division. The shales occur chiefly in the vicinity of the coal seams and are generally f inely textured rooka with a slaty cleavage. The Tantalus conglomerate overlies, to a l l appearance, conformably, the Laberge series. Fossil (69) (1) plants from theeoal seams place the age of the con-glomerate as Cretaceous. (2) Fossil plants collected by Caimes from the Wheaton distr ict placed the age of the conglomerate as Jurassic. In view of the fact that the beds were 11th-ologically similar to the Kootenay beds and contained coal seams and that some forms of the foss i l plants had been reported from the Kootenay formation, Caimes classed these beds as probably belonging to the Cretac-(3) +ous. Cockfleld and Bel l , however, consider that; "The plant evidence given above seems to corroborate the animal evidence obtained from the Laberge beds, and there can be l i t t l e doubt that the Laberge beds and Tantalus conglomerate are both of Jurassic age, and i t would appear that the ear l ier determinations of foss i l s are to be regarded as extremely doubtful in the light of the more exact evidence now presented." Cretaceous. Rocks of this age have been described from (4) the Upper White River d istr ict , they consist to a great extent of dark or banded shales and arg i l l i tes with which are interbedded a large proportion of greywacke ( l ) Caimes D.D. "Lewes and Nordensklold Rivers Coal d i s t r ic t , " G.S.C. Mem 5^ .38 . 1910. (2) Caimes D.D. G.S.C. Sa*. Rp. 1915 P.41. (3) Cockfleld W. E. and Bell A.H. "Whitehorse d i s t r ic t . " G.S.C. Mem 150 P.23 1926. (4) Caimes D.D. "Upper Whit* River d i s t r ic t , " G.S.C. Mem 50 P.84 1915. (70) and smaller amounts of conglomerate and sandstone, the entire series being notably more siliceous than the underlying Carboniferous beds. (1 ) Similar rocks were found by Calmea along the Yukon-Alaska boundary. The lithological descriptions are similar. These sedimentary formations were found to l i e conformably upon the Perms-Carboniferous (?) beds and in places i t was d i f f i cu l t to determine between them. Fossils, although rare, were found and they place the age of the beds as Lower Cretaceous. Formerly the Laberge series and the Tantalus conglomerate were considered of Cretaceous age but the Laberge series i s definitely of Jurassic age as shown. (2) The Tantalus conglomerate Cockfield thinks may be younger than Jurassic but i t rests conformably upon the Laberge series and foss i l evidence, so far , appears to indicate a Jurassic age. (1) Caignes D.D. "Yukon-Alaska Boundary," G.S.C. Mem 67, PP. 105-106 1914. (2) Cockfield W.B.Personal Communication 1930. (71) TERTIARY Tertiary sedimentary rocks have been described from a few areas namely the Upper White river, Sixty-mile river, Kluane and Elondyke distr icts . They have also been described from the Porcupine, Peel and Frances rivers. These beds comprise mainly loosely or only part-ly consolidated sandstones, shales clays and conglomer-ates. The sandstones are prevailingly greyish to yellow-ish and brown in colour, and the shales and clays are dominantly some light shade of grey, green or blue, but some quite black strata occur. The shale is a dense, compact rock containing abundant f o s s i l wood, though no other foss i l s could be found. The sandstone is a soft , f r iab le , coarse sandstone or arkose, consisting of quartz and decomposed feldspar with abundant ferruginous matter. These sandstones contain a few scattered pebbles of quartz, quartzite, or schist, and towards the upper por-tion of the beds the pebbles become more numerous, the rock grading into a conglomerate. Where these beds have been intruded by volcanic rocks they have become indurat-ed probably due to the inf i l t rat ion of siliceous mater-ia l s from the volcanics. A l l the beds are so f t and decrepit , tc readi ly to forr: sand and clay beds. Tha^e ree ls ^re pi-iv,.ijJngly (72) f l a t lying, and in most places have been only slightly disturbed by earth movements. They have been extensive-ly invaded by more recent volcanies including members of both the rhyolite- latite group and the newer volcan-ics , which pierce or overlie them wherever they are exposed. Their occurrence in scattered patches on the upland suggests that they participated in the movements of the Yukon Plateau prior to i ts planation. ( I ) Prindle is of the opinion that the lower beds of these sediments are usually fine-grained but become coarser towards the top and the upper beds are almost always conglomeratic. The l ignite does not appear to be confined to any particular horizon in these beds. Though the absence of foss i l s in these beds makes their age d i f f i cu l t to determine, their structural relations show they participated in the movement of Yukon plateau prior to i ts up l i f t and subsequent plana-(2) tlon. Spurr believes that the up l i f t and planation of the Yukon plateau was probably contemporaneous with the deposition of Miocene strata in lower valley of Yukon river, and that the beds are Pre-Miocene. These rocka are probably continuous with the Tertiary rocks described (1) Prindle L.H. "The Forty mile quadrangle," U.S.G.S. Bull . 375, 1909 PP. 23*26. (3) Spurr J.E. "The geology of the Yukon gold d ist r ict , Alaska," U.S.G.S. An.Rp.Pt.3,1898, PP. 260-263. (73) (1) by McConnell from the Klondike region as the Kenai aeries and possibly equivalent to certain beds in the Fortymile district which, from foss i l evidence, have (2) been referred to-that age. These correlations place the rocks as Upper Eocene. Since, also, the Kenai beds contain seams of l ignite, i t i s customary to include in that formation a l l Tertiary beds containing coal. I t would thus seem possible that rocks more recent than Eocene have bran in places Included in the Kenai aeries. The Kenai sediments in most places represent deposits laid down in separate basins of deposition, and the plant remains which they contain show that most of them at least are of fresh-water origin. The lignite seams in the Kenai series are not confined to any particular horizon, but occur in a l l positions from top to bottom of the series. This i s just what might be expected, considering that the Kenai beds are be-lieved to have been deposited in unconnected basins, in which case, the coal seams would not be formed in a l l the basins simultaneously, nor would they occupy similar positions in the series in different loca l i t ies . (1 ) McConnell R.G. G.S.C. An.Rp. 1901 pp. 23B-24B. (2 ) Prindle L.M. Op.Cit. pp. 23-26 (74) QUATERNARY and RECENT. These deposits are very extensive in Yukon and in places quite thick. They are generally con-fined to the valleys and lowlands; as a rule they are lacking on the upland surfaces. These deposits are of g lac ia l , f l uv ia l , and lacustrine origin and consist of sand, gravel, so i l , Kilt, clay, boulder clay and a subordinate amount of volcanic ash. (75) IGNEOUS ROCKS. Pyroxenite and Per ido t i t e . Under th is heading a l l Pre-Mesozoic igneous rooks w i l l be described except those already described in connection with the Yukon group* These Pre-Mesozoic Igneous rocks are comparatively wide spread and vary great ly in composition, with possibly basic rocks pre-dominating* ( 1 ) Cairnes describes a ser ies o f igneous rocks along the Yukon-Alaska boundary as to include d i o r l t e s , andesites and diabases. These occur as dykes, s i l l s and small intrusive masses* They appear to be exten-s ive ly developed in association with the sedimentary members of the T indir group (Pre-Middle Cambrian). These rocks, however, have not only invaded the T indir sedim-ents but in addit ion they have intruded the members o f the Yukon group, as we l l in places as the lower beds o f the Devono-Carboniferous l imestone-dolomites. A l l the intruslves examined in this regior proved to be diabases although Cairnes states that other re lated roek types may be present. Cairnes places the i r age as ranging from Carboniferous to Pre-Cambrlan but thinks they ere domiaantly Pre-Middle Cambrian., ( 1 ) Cairnes b.D. "Yukon-Alaska Boundary," G.S.C. Mem 67, P. 109 1914. K f (76) These diabases are preva i l ing ly greyish to dark green, f i n e to medium textured rocks which possess an ? - * ophit ic structure, and may or may not be amygdaloidal in character. When amygdaloidal the anrygdules are domlnantly f i l l e d with secondary minerals mainly quartz, c a l c i t e , z e o l i t e s , or ch l o r i t e . On weathered surfaces these intrusives are charac ter i s t i ca l l y reddish to red-dish brown, due to the oxidation o f the iron-ore miner-als which they contain and which in some o f these rocks are somewhat abundantly distr ibuted or peppered through the rock mass. (1 ) Cockfie ld describes a ser ies of igneous rocks from theUpper Beaver River area. These rocks are both intrusive and extrusive , but occur ch i e f l y as s i l l s . Two types o f igneous rocks are found in this l o c a l i t y , one an augite andesite, the other an augite d l o r i t e . The augite andesite, i s a dark green, f i ne to aphanitic textured rock which under the microscope shows advanced a l tera t ion to secondary minerals. This rock occurs both as an intrusive and as an extrusive . The augite d l o r i t e varys from coarse to f i ne texture but i s o f g ran i t i c habi t . This rock contains augite and andesine. The augite andesite and augite d l o r i t e are considered of post Ordovician-Devonian age since they are found (1) Cockfield W.E. G.a.C. Sum. Rp. 1924 Pt. A (77) intrusive into Pre-Ordovician-Devonian rocks. Although Cockfield found no evidence, he thinks that the andesite and diorlte are o f the same age. Pyroxenlte and peridotite rocks are described (1) (2 ) from the Mhitehorse and the Wheaton distr icts . Associated with these basic igneous rocks are veins f i l l -ed with serpentine and, in some cases, coarsely c rys ta l -l i n e amphibole. Talc and magnesite were also found associated with'these ve ins. The serpentine i s dark o l ive-green with a waxy lustre and " pale bro.vn coating of weathered products. Some serpentine veins have cent-ra l veins of chrysot i l e asbestos. Fibrous serpentine and ta lc occur in narrow f i ssures throughout the rock and on surfaces which appear sl ickensided. The pyroxentite and per ido t i t e rock as a whole i s very massive. Fresh surfaces in some cases appear coarse-grained granular, and in others no granular texture i s v i s i b l e . The rock i s medium to dark grey or black, with, in places, a greenish t inge . The weathered surface i s coarsely p i t ted and has a bright reddish-brown colour due to the presence of i ron oxide. Under the microscope the texture of the basic rocks was seen to be hynldiomorphic granular and rather coarse grained. The rock contains over 80 per cent and 11) Cockfield W.E. and Rell A.H. "Whitehorse d i s t r i c t , " G.S.C. Mem. 150 P. 10 1926. (2 ) Caimes D.D. "Wheaton d i s t r i c t , " G.S.C. Mem. 31, P. 49, 1912. (78) up to 95 per cent o l i v i n e . Chromite i s prominent as an accessory* probably making up 2 per cent o f the rock* Serpentine i s an important al ternat ion product. The contact o f these rocks with other fonn-i l ) ations was not seen. Cockfield i s o f the opinion that the per idot i t es may be in t rus ive . He corre lates (2) them with the *Gold Ser ies* . Calmes was unable to obtain in the Wheaton d i s t r i c t any d e f i n i t e information concerning the age o f these rocks except that they cat members o f the Mt Stevens group (Yukon Group), but from the i r l l t h o l o g i c s imi lar i ty to the rocks in other parts o f Yukon and in northern Br i t i sh Columbia, he considers them to be probably o f about Devonian age. Older Volcanics. The rocks of the Older Volcanles group are ch i e f l y andesites, diabases, and basa l ts . Smaller quantit ies o f deep-seated, basic rocks such as d i o r i t e , gabbro and amphlbollte have been included. Areas o f the Older Volcanics are rather wide-spread thoughout Yukon and have presented somewhat o f a problem as r e -gards the i r age. These rocks are t yp i ca l l y compact, f i n e l y textured, and dark green, but red, brown, and blue types also occur. They are preva i l ing ly porphyr i t ic , with fe ldspar crysta ls in an aphanitic ground-mass. In seme (1 ) Cockf ie ld W.E.& Be l l A.H."Whltahorse d i s t r i c t , "G .S .C . Mem. 150 p.11 1926 (2 ) Caimes D.D. "Wheaton district,"G.S.C.Mem.31 1912. (79) cases phenocrysts of hornblende and biotlte may be discerned with the naked eye* Iron in the form of magnetite or pyrite is commonly present, and in many oases has oxodized, giving reddish or brownish colour to the rocks* Tuffs and breccias occur in may places. Under the microscope these rocks are seen to possess a var ie ty of compositions and of structures. Plagioclase i s always present and general ly occurs in two generations. I t ranges from o l i goc lase to bytownite in composition, but by f a r the :.ore common r lag ioc lases or andesine or labrador i te . The acid p lag ioc lase i s present ch i e f l y in the groundmass of the rocks. Orthoclase occurs in a few cases as phenocrysts, and also in the groundmass. The ferro-magnesiuxi minerals include hornblende, b l o t i t e , pyroxene, and o l i v i n e . Both the common green hornblende and brown basa l t i c hornblende occur, but the former i s b,. f ^ r the more c common. B lo t i t e i s also common and in some cases i s the only ferromagnesium mineral present. B i o t i t e and hornblende occur together, and ex is t in both genera-t i ons . Pyroxene, usually d iopside, i s present, but seldom in phenocrysts o f su f f i c i en t sise to be detected with the naked eye. Ol iv ine has been noted in some of the augite andesites. Pyr i te and magnetite are abun-dant, in many cases in specks large enough to be detect-ed with the naked eye. (80) The a l te ra t ion of these rocks in some cases i s we l l advanced, and in many cases masks the o r i g -ina l character. Ca l c i t e , ch l o r i t e , epidote and z o i z l t e are abundant as secondary constituents. The structure of the rocks i s usually prophy-r l t i c , and the phenocrysts, as described, consist of plagioclase and the ferromagnesium minerals. Pheno-crysts are as a rule f a i r l y abundant. The groundmass i s e i ther ho locrysta l l ine or part ly g lassy. The rocks o f deep-seated character that have been included under the term "Older Volcanlcs" are on the whole greenish in colour, massive and from f i n e -to medium-grained. These rocks Vary in composition but are general ly high in ferromagnesium minerals. Zo i z i t e and serpentine are common as a l te ra t ion pro-ducts. ( 1 ) In h is e a r l i e r work Calmes separated the "Older Volcanics" Into two groups, the Perkins group and the Chieftain H i l l vo lcanics . This subdivision ( 2 ) was l a t e r abandoned and both were included In one group and correlated with the Older Volcanics o f White (3 ) River d i s t r i c t . Portions of the Older Volcanics are d e f i n i t e l y intrus ive into the Laberge beds, and may, (1) Calmes D.D. "Wheaton d i s t r i c t , " G.S.C. Zei'i 59-64 19*12 (2) Cairnes D.D. G.S.cl Sum Hp. 1915* P. ( " ) Cairnes D.D. "Upper Vihite d i s t r i c t , " G.3.C. HetL.50 P.P7-9C. (81) there fore , be considered as younger. The s imi lar i ty between the t u f f s of the Laberge beds and those o f the Older Volcanlcs has been pointed out by Calmes, who bel ieved th&t the period of vulcanism represented by the Older Volcanlcs was in some measure contemporaneous ( 1 ) with the deposition of the Laberge beds. Gwilllm i s o f the opinion thot these rocks may be c lose ly connect-ed with the o r i g in of the sandstones. He also noted that "the change from t^o fragmental rocks to the por-p h y r i a s and andesites i s gradual . " The older Volcanlcs, are, probably, a l l o lder than the gran i t i c intrusivos. Although some of the f lows arc -ore recent than the Laberge ser ies with which ( 2 ) - they come in contact,Cockf ie ld i s of the opinion that they are to a large extent contemporaneous with the Laberge scr ies and are probably of Lower and Middle Jurassic age; and that the t u f f s , which are so p l en t i -fu l in tho Labrrge ser i es , are to be attr ibuted to the same period o f vulcanlsm. Granit ic Intrus ives . The grani t i c Intrusives form one of the major geo log ica l formations of Yukon, both in areal extent , and in importance as possible ore-br in^ers. These rocks (1 ) Gwillim J.C. "AtlJn Liining d i s t r i c t , " G.S.C. An. Rp. Vol X I I Pt . B. P. 28. (2) Cockfield and Be l l A.H. "^hiteborse d i s t r i c t , " G.S.C. Mem. 150 p. 29 1926. (82) are exposed mostly in southern Yukon where the Coast Range mountains trend northwesterly across the southern portion o f this province. Considerable g ran i t i c out-l i e r s have been described but i t i s the general opinion that these i so la ted outcrops are connected with the main body of the in t rus ive . Thus i t i s the consensus of opinion that the Coast Range intrusivos probably underlie at l e a s t , the southern portion of the Yukon province. The rocks grouped under the head of Coast Range intrusivee present many d i f f e r e n t types, but they have in general a g ran i t i c hpblt . They are general ly grey in colour, and f resh and unalteraa in appearance. In some c .ses , however, pink fe ldspar i s present in su f f i c i en t quantity to g ive a pinkish cast to the rocks, but on the whole th is i s except ional . The typ ica l rock of the Coast Range intrusives i s medium to coarse-grain-ed, with the essent ia l constituents v i s i b l e to the naked eye. Locs l l y , phenocrysts o f fe ldspar , many exceeding one to two inches, ere developed, and the rocks may be said to have a porphyrit ic texture . Quartz, orthoclase, p lag ioc lase , ond ferronagnesiun minerals, in nearly ever;, case hornblende and b i o t i t e , can readi ly be ^?tect-ed by the unaided eye. When examined under the microscope, the major-i t y of the sections are seen to contain quartz, ortho-(83) clase, microcl ine, p lag ioc lase , hornblende, b i o t i t e and, only in some cases augite. The amount of quartz var ies g r ea t l y , but i s mostly between 10 and 25 per cent fe ldspars from 60 to 75 per cent, and the remainder i s hornblende and b i o t i t e , or augite* Orthoclase and mic-ro l lne are as a rule about equal in amount to the plag-i o c l ase . Plagioclase i s most commonly o l i goc lase or andesine. The hornblende and auglte^ in many places, are intergrown. B io t i t e i s present as a ru le . The t yp i ca l rock would, there fore , seem to have a composition midway between granite and quartz-d i o r i t e . To this the name granodiorite has been applied also i t may be termed a quartz-monzonite. With an increase o f the orthoclase and decrease of the p lag ioc lase , typ ica l granites have been noted, and t . e decrease o f orthoclase and increase of p lag io-clase and augite g ive r i se to d i o r i t e s . An attempt was ( 2 ) made, in the Alshlhlk L<rke d i s t r i c t , to determine whethe there was a progressive change in composition across the bathol i th or along i t s s t r ike , but the conclusion was reached that the var iat ion from the normal type around any s ingle l o c a l i t y was greater than the var ia t ion ex-h ib i ted by the normal type e i ther across the bathol i th or along i t s s t r ike . Ko regular change in the normal (1) Cockf ie ld W. E. and Be l l A.H. Opl c i t . (2 ) Cockfield W.E. Sum. Rp. Pt .A. P.6A 1926. (84) type of intrusive was detected, but i t must be remember-ed that the batholith in this region i s narrow, and possibly no striking change is to be expected. ( 1 ) Caimes describes these rocks as "under the microscope decidedly granodlorite," while he describes (2) somewhat similar rocks from the Upper White river as commonly diorites but "in addition, granodiorites, quartz granodiorites, granltites, gabbros, and even homblendites were examined." Where* the bathol i th i s in contact ^ i th the schistose rocks o f the Yukon group, as occurs in many instances, i t cannot be determined what part of the metamorphlsm of these rocks i s due to the intrusion o f the bathol i th , and what part i s due to other causes. Along many of the contacts garnet i s found in the schists , pointing to a certain degree of contact metamorphlsm. Where the limestones o f the schistose group are in con-tact with the granites the defree of contact metamor-phlsm i s high; garnet, eipdoto, and other s i l i c a t e minerals are abundant and in some cases make up the bulk o f the intruded rock, but the zone where these minerals are found i s quite narrow. In no instance have s i l i c a t e minerals been noted more than a quarter of a ( 3 ) mile from the gran i t i c contacts. (1) Caimes D.D. "A t l in d i s t r i c t , " G.S.C. Mem. 37, P 58, 1913. (2 ) Caimes D.D. "Upper White River d i s t r i c t , " G.S.C. Mem 50 p. 95 1915. (3 ) Cockfield W.E.G.S.C. Sum.Rp. P.7A 1926. (85) (I) Cockfield found that the western contact of the bath-ol ith where observed, in the Desadeash distr ict , was steep but the contact minerals show i t to be gently sloping* Pegmatite dykes which are usually contact phenomena have not been reported from the eastern con-tact but although not common do occur along the western contact. These pegmatite dykes general ly contain large crysta ls of tourmaline. (2) Cockfield has revealed some interact ing f ac t s as regards the, Coast Range ln t rus lves . Here he notes that ; "the presence, on the tops of many of the higher h i l l s , o f bodies of the intruded rocks, leads to the bel ief that these are possibly remnants of the roof of (3 ) the bathol l th . In Whe^ton d i s t r i c t sever* ! long, re-l a t i v e l y narrow curtains of Pre-Jurnseic rocks occur in the bathol l th , are cut by the va l l eys to depths of 3,000 f e e t or more, and at the l e v e l of the va l l ey bottoms are almost as wide as at the i r highest j o i n t s . In the same d i s t r i c t , small, I r regu lar ly shaped patches of the older rocks outcrop at widely d i f f e r en t e leva-t ions . These cannot be ports of the roof o f the bath-o l i t h but are inclusions. The method of batho l i th ic (1 ) Cockfield G.'^.C. Sum Rn. pprt A 1927 P.6A. (2) Cockfield R'.E G.S.C. Sum.Rp. P. 8A 1926 ( 3 ) Cairnes D.D. "Wheaton d i s t r i c t , " G.S.C. Men. 31, PP 74-76 1912. (86) invasion which best answers the known facts appears to be that o f overhead stoping — the bathol i th advanc-ing by the breaking away from the roof o f fragments or blocks which sank in the magma that rose to replace them. The bathol i th also appears, in general , to have intruded the overly ing rocks in the form of ^reat tongues and dykes, from "hlch branched o f f smaller por-t ions . However, there in ^ot the minute in te r f inger lng of the bathol i th and the o lder rocks that occurs in connection *vith the Pro-Cambrian bathol i ths . There i s l i t t l e in the way of addition o f g ran i t i c mater ial to the intruded rocks, except in the form of d i s t inc t bodies such as dykes and s i l l s . That p certain amount seems evident, f o r at the contacts of seme of the dark-er rocks the granite becomes darker rs the contact i s approached, but th is i s operative f o r ^ few f e e t on ly . " i t i s almost certain that n cover was main-tained over the ^agmp unt i l i t cooled. Further, i t would appear that the reef of the bathol i th was highly i r r egu la r . This i s perhaps, be?t i l lus t ra ted on the h i l l s north of Champagne, vhere the submits of granite and, there fore , r?ct at the roof of the bathol i th ; passing northwird the s^- is t contact a ^e^rr rt a dlst in^e of 2 ir l les rnd ?t ?n e levat ion of Ft l eas t 2,000 f e e t lower, s t i l l far ther north^^rd, schists are exposed in the bottor of the Mendenhall v s l l e y ; and on (87) the higher h i l l s to the north of this va l l ey granite once more appears. I f i t be assumed that these granite bodies are connected beneath the schist cover, the or-i g i na l roof of the bathol l th must, indeed, have been highly i r r egu la r . Moreover, a study o f the areal geo-logy of At l in to 'Ahitehorse shows that there are numer-ous outlying bodies of granodiorite thab are ident i ca l In appearance and composition .vitt the intrusives of the bathol l th I t s e l f , and no evidence has as yet been presented to show thct those ;;rc of a d i f f e ^ ^ t age. They can, perhaps, best be regarded as peaks of the gran i t i c bod^, .hlch have, as ye t , been barely deroofed. " I t appears to the wr i ter than i f the Pre-bs thc l l th lc r o d s occur as "curtains" as stated by Cockf ie ld and i f the g ran i t i c intrusives have assumed the form of huge tongues cn intrusion, these might account f o r the apparent regu lar i t i es described from Cockf ie ld above. I t seems quite possible th-t these "curtains" could eas i ly be tuo or ^cre Jiiles in width or that there could, be a distance oi two, or j o ss ib l y more, miles between the ton^ue3 o f the in t rus ive . Cockfield in h is report on the sar.e d i s t r i c t , has shown that the mincr-1 deposits g ive some indica-t ion o f the shape of the b^dy of intrusive rocks. He f inds that there are t.;o ty^es o f mineral izat ion, namely contact metamor, hie s.nd hydrothermal. Con.ta.cb metamor-(88) phic deposits are bel ieved to have been formed under conditions o f high temperature and pressure, and con-sequently soon a f t e r the intrusion of the batho l i th . The hydrothermal deposits of Yukon, belong to deposits formed under conditions of moderate temperature and pressure. As these occur not only in the surrounding rocks, but in the granodiorite i t s e l f , i t fo l lows that these were formed at a l a t e r d^te than the contact met-auiorphic deposits, a f te^ the up^er part of the grarodlo-r i t e had s o l i d i f i e d and cooled. "Moreover, as the mineral deposits of this region occur in a be l t fo l lowing the eastern nargin of the bathol i th , and as the deposits have been found to be genet ica l l y connected with tha bathol i th , i t fo l lows that the roc^s to the east of the main boundary ("where most of the Lydrothermal deposits occur") are r ea l l y the roof of the batho l i th . This i s further borne out by the fac ts that numerous outlying bodies of granodiorite occur to the ¿ast o f the main margin of the intrusivos and no evidence has yet Leen found in th is region that these outlyin^ bodies d i f f e r in age from the main intrus ivos , lb fo l lows bhat on the whale the eastern margin o f the bathol i th in this region slopes gently eastward with re-current upward project ions ¿¿hose summits have been l a i d bare to the east of the main margin. This conclusion — * ^3 (89) / ( 1 ) not agree with that o f Schof ie ld , namely a steeply-dipping and ^mooth*flowing eastern contact, with a ,' t narrow ^oat#ct metamorphic zone. / "The deposits o f the contact met amorphic type occur f o^ the most part at considerable distances from the main margin of the Lathol i th and are found at the borders o f out ly ing bodies o f granodior i te . Deposits o f this type, with one possible exception, are confined e i ther to limestone, schist or granodior i te . At one ( 2 ) l o c a l i t y , Jjoeker creek, a deposit of this type occurs in a schist inclusion, and deposits of hjdrothermal or ig in occur in the granodiorite at approximately the same e levat ion . As the hydrothermal deposits ^re low temperature types, compared with^ the contact mctamorphic, and as the d i f f e rences in th is case c nnot be explained by zoning as ordinar i ly understood, i t , there fore , appears that the time at which the deposits were formed becomes the deciding factory that deposits formed soon a f t e r the intrusion of the bathol i th were of the contact metamorphic type; and that near them may be found de-posits of the upper vein zone formed in the dying stages of vulcanism irom the same intrusion.^ The age of the Coast Range intrusives has seen, t l ) Schofield S.J. and Hanson G. "Salmon River d i s t r i c t , " G.S.C. Mem 132 pp. 64-66 1922. (2 ) Cairnes D.D. "Wheaton d i s t r i c t , " G.S.C. Mem 51, PP. 110-111, 140-145 1912. (90) and s t i l l i s , the source o f many discussions. áfhen ( 1 ) Calmes f i r s t worked in the Wheaton d i s t r i c t he was o f the opinion that the intrusivos antedated the Mesozoic (2 ) (3 ) Laberge beds, but his l a t e r work in A t l in and Wheaton d i s t r i c t s corrected this when he rccognized the f a c t that the granite cut the uppermost of the L^berge beds Lnd even the Tantalus conglomerate. A prominent feature of the Labcrge beds i s con¿_lom.. r - tos -^hich contain pebbles and boulders of . jytni t ic rocks ident i ca l in character with .the Co.^t Ka^ge intrus ives . Trom this d-ta Cairnes concluded thut the Coast Range in t rus ives , although l i tho lo^ ica j - ly very s i ^ i l ^ r , "¿ero intruded at d i f f e r e n t t i^es ; that p^rt& of Lhe bathol i th v.ere intrud-ed and deroofed to supply material f o r the Labcrge beds, ¿jid that_this period of sedimentation w^s fol lowed by further intrusion. Nothing, however, ha? been ^ut forward to show th^t these pebbloa and boulders were actually derived from the Corst Range batho l i th , except the i r l i t h o l o g i c a l i ' lr^i lority to the intrusives nov.* found. For purposes of a^e determinations, this evJdcncc i s la rge ly va lue less . I t in, there fore , Mere reasonable tc the Coast Range intrusives ir.ith respect only to the rocks they cut. , ITT Calmes' L.D. "Y.heaton d i s t r i c t ' ' C.&.C7"SeF31, P 53, 1912. (2 ) Calmes L.D. "Ablln d i s t r i c t , " G.L.C. Hen 37,P.59,1913. (3 ) Calmes D.D. G.S.C. Sum. Rp. 1915 P. 42. (91) The l a t e r determination of the age of the Laberge beds as Lower and early Middle Jurassic l im i ts to some extent the age o f the intrus ives . They are not e a r l i e r than the lower part of the Middle Jurassic. With regard to the up;er l im i t o f the i r age the evidence i s less cer ta in. I t has not been d e f i n i t e l y established that the Upper Jurassic i s not represented by some part o f the Laberge ser i es , and the age of tne Tantalus con-glomerate remains in doubt. Caimes i s o f the opinion that the Tantalus conglomerates are o lder than the intrus ives . He correlated the Tantnlus conglomerate with the Kootenay formation o f Lower Cretaceous age (1) found in northwestern Br i t ish Columbia. But Knowlton, a f t e r examining one co l l ec t i on of f o s s i l plants from the Tantalus, pronounced them to be Jurasaic. Thus the evidence obtained to date in Yukon indicates that the granite i s more recent than the lower part of the middle (2) Jurassic and older than certain Tert iary rocks. Beyond th is the age i s not f i x ed . Newer Volcanics. This includes an important groun of volcanic rocks comprising mainly andesites, basalts , anascct^te t u f f and breccia and scor ia . These rocks are wide spread over Yukon and have been reported on from many l o c a l i t i e s . These rocks ^resent, characteristic;; l l y , a bright (1 ) Cairnes D.D. G.S.C. Sum. Rp. 1915 P. 41 * (3 ) Cockfield '*.E. G.S.G. Sum. Rp. 1927 P. 6A. (92) f resh appearance and are contrasted in th is respect with the dominantly dul l appearing o lder volcanics pre-viously described. Black, and various shades green and grey, predominate, but reds, ranging from a dul l br ick-red or even purple to bright vermi l l lon or even lavender, are by no means rare . Black scoria i s abundant in the Ihltehorse d i s t r i c t . The t u f i s and breccias^ which are preva i l ing ly l i gh t e r in appearance than the lavas with vhlch hhcy are IntorbedJod, are general ly ash coloured or aP.aic ^ol gra^, or j ^ l l ow . These ¿xtv-aive l^vas aro f o r the greater part porphyrit ic rock& of medium coarseness, containing phen-ocrycts o f p l ^ i o c i a s e , hornblende, pyroxene, h i o t i t e and o l i v i n e . The pl^glcelase fe ldspars range f r o m o l i -goclase to by to jn i t e , but are usually andesine, labra-duri te , or bytownite. Both common green hornblende and basa l t j c hornblende occur. The pyroxene i s mostly diop-side or hypcr^thene. The chief a l terat ion products are ca l c l t o , e^idot^ ^nd ch lo r i t e . ( 1 ) These lav-n in the dp^er ^hlte River d i s t r i c t cut and over l i e the Tert iary sediments and are thus at l e Joccne in a^e. fnese rocks have been studied in a number of l o c a l i t i e s la Yukon, and wherever d e f i n i t e determinations ¿ould be made they have been assigned to ITT^Cairncs .siiiti River Mem 50. P. 100 1915. (93) the late Tert iary or early Pleistocene. They have been found to cat the Older Volcanics and the Coast Range intrusives, and are In turn intersected by the Acid (1 ) Volcanics. Mendenhall, who studied similar f lows in Alaska, says, "These f lows, therefore, instead of pre-ceding the deformation of the early Tert iary p la in , are l a t e r than the dissect ion which followed i t s u p l i f t , and are to be regarded as very recent indeed." In Whltehorse d i s t r i c t i t i s certain that at least some of the flows have been poured out since the v .Hoys were cut approximately to the i r i r e smt dejths. This i s apparent in the c^se of F i l es canyon, "rhore Le es r i v e r has cut through a basalt f l c ? in the v r l l e¿ to a depth of probably less than 100 f e e t . Acid Volcanics. Grouped under this heading i s a series o f rhyo-l i t e s , quarto-porphyries, granite-porphyries, l a t i t e s and related rocks with accompanying t u f f s . These rocks represent the most recently consolidated roc.is in Yukon and are of wide occurrence throughout the d i s t r i c t . They occur ch ie f l y as dykes and small rassos. The members o f this group consist of l i gh t coloured porphyritic rocks ranging frou nearly .b i te , l i gh t grey or yel lowish, to pale lavender or d-rker green-t l ) Mendenhall ^.C. "The geology of the Úa'n^ral Copper River Region, Alaska,"U.S.G.s. Prof.Paper 41 PP. 54-62 1905. (<j) Cockfield &Bell A.H.*Whitehorse d i s t r i c t , " G.S.C. Mem. 150. P. 33 1926. (94) ish grey shades. The groundmass is invariably crypto-crystalllne and the phenocrysts include mainly feldspars, hornblende, and biot ite . These yoeks have been describ-ed as rhyolltes, lat i tes , quartz-porphyries, granite-porphyries andesltes, basalts, and other varieties have been noted. They are thought to have been extruded upon the surface through fissures and necks, but the fissures were not open over great distances as these lavas only occur local ly . ( 1 ) Cockfield i s of the opinion that the andesitic and basaltic flows are older than the more acid variet-( 2 ) ies . Caimes, in the Wheaton distr ict , observed volcanic necks, surface flows, and tuffaceous accumulations in their characteristic forms and concluded that these rocks were of comparatively late origin, and assigned them to the late Tertiary or Pleistocene. In the Mayo h i l l (3 ) (4) d istr ict , Stockwell and Cockfield in the Aishihlk Lake district consider that these rhyolltes and quartz-por-phyries are connected with the granitic rocks. These Acid Volcanics cut the Newer Volcanies wherever they come in contact with them and are thus younger than the Newer (5) Volcanies. In the Up.er White River distr ict Caimes (1) Cockfield W.B. G.S.C. Sum. Rp. 19H2 P.6A. (2) Caimes D.D. "Wheaton d ist r ict , " G.S.C. Mem 31, P. 60-76 1912. (3) Stockwell G.S.C. Sum. Rp. 1925 P.A. (4) Cockfield W.E. G.S.C. Sum. Rp. 1926 P.A. (5) Caimes D.D. "Upper White River d is t r ict , " G.S.C. 50 P.101 1915. (95) shows that these rocks have flowed over the present land surface since i t has become uplifted and eroded to nearly i ts present form, the topographic features having since been modified only by glaciation. They are thus of late Tertiary or Pleistocene age. . 3 UBC Scanned by UBC Library (96) ECONOMIC GEOLOGY. The economic aspect of geology in Yukon i s not very favourable. In the f i r s t place prospecting i s ex-ceedingly d i f f i c u l t due to the thickness o f the d r i f t and the overburden. In the seeond place, Yukon i s rather remotely situated and thus the cost o f production i s great ly increased by the transportation charges. Although the climate o f the region i s not as severe as considered by most people i t i s severe enough to prevent operations f o r approximately seven months, however, the long_days In the summer months readi ly ?ermit surface work without the use o f a r t i f i c i a l l i g h t s . The mineral deposits consist o f p rac t i ca l l y only two types, namely, contact metamorphic deposits and f i ssure vein-replacement deposits. The l a t t e r i s d i v i s i b l e into three d iv is ions and w i l l be dealt with in de ta i l l a t e r . The contact metamorphic deposits contain ch i e f -l y copper values with traces of gold in some places. They occur as i r regular masses without any d e f i n i t e trend. They occur in two conditions; f i r s t in a l tered limestone close „to or in d i rec t contact with the Coast Ranfie intrus lves , and secondly, they occur in the schis-tose rocks at the contact with the same intrus lves . Cockfleld states," There can be no doubt about the close genetic connection between the ore-bodies and the intrusion of the Coast Range batholith, this connection being apparent even on most casual examination." The minerals are chalcopyrite, bornite, tetrohedrite, chal-cocite and their oxidation products, malachite, azurite, chrysocolla, cuprite and malaconite. The cuprite is occasionally associated with native copper. Pyrrhotite and pyrite are not abundant but magnetite and hematite occur in large masses. The ores have either a gangue of magnetite and hematite or a gangue of si l icate minerals. The gold-si lver veins are typically high temper-ature deposits in the intruded rocks and in the granite i t s e l f . The gold occurs in the free state and is intim-ately associated with the sulfides, pyrite and pyrrhotite. These deposits are of considerable importance as the sourees of the placer gold of Yukon but not at present of any great economic value. The si lver- lead veins represent the mining wealth of Yukon. These veins are subdivided into three (2) groups on a mlneralogical basis. In the f i r s t group galena and friebergite are the chief ore minerals and manganiferous siderite is the chief gangue mineral. Quartz and pyrite are present in small quantities. Cerusslte, limonite and manganite are (1 ) Cockfield W.E. G.S.C. Sum. Rp. 1922 Pt.A. (2) Stockwell G.S.C. Sum. Rp. 1925 Pt.A. (98) oxidation products; chalcopyrite, malachite and azurite are present in small amounts. These veins are high in si lver values with no gold values. They represent the best mineralization type of vein in Yukon. The second group of veins have a gangue of quartz or ankerite, calcite in a few places, and side-rite either subordinate or absent* The ankerite usually contains manganese. Galena, frieberglte and zinc-blende are the ore minerals; pyrite, limonita, cerussite, chal-copyrlte, malchite and azurite are also present. In these veins the si lver content is important although small values for gold are obtained. This group of veins and the group described immediately above belong to the transverse veins which wi l l be described later . The quartz-arsenopyrite veins represent the third group. Quartz is the chief gangue mineral but ankerite, calcite and sericite are present. Arsenopyrite is characteristic and pyrite, galena, zinc-blende and pyrrhotite occur also. Native gold has been found in these veins, but like the si lver values, is low. Cerus-site^ and limonite occur as oxidation products. Deposits of this type belong to the longitudinal class, as des-cribed below, and are not as yet of any importance. (1 ) (2) Cockfield and Stockwell have described the lead-(1 ) Cockfield ^.E. G.S.C. Sum. Rp. 1923. P t r U l (2) Stockwell G.S.C. Sum. Rp. 1925 Pt.A. (99) s i lver veins in some detail in the Mayo distr ict . Here they found that the ore deposits are prac t i ca l l y a l l f i ssure ve ins , that i s , they represent vein material deposited in f au l t f i s sures . The fau l ts are a l l normal f au l t s with a horizontal displacement o f 500 f e e t or more. The longitudinal and transverse f au l t s occur and are determined mainly by the i r mineral izat ion. The transverse f au l t ing , that cutting across the bedding, i s attr ibuted to the bending o f the beds from an east-west d i rec t ion to a southerly d i rec t ion and they trend o ó N. 5 W to N. 15 E. The longitudinal f au l t s , those pa ra l l e l l i ng the str ike o f the beds, may be due to the stresses developed at the time of the intrusion o f the o o gran i t i c masses and trend N. 30 -40 E. The e a r l i e r mineral izat ion, in the longitudinal f au l t s , consists o f quartz, arsenopyrite and pyr i t e * These longitudinal f i ssures remained planes o f weakness, a f t e r f i l l i n g , and were a f f ec ted by subsequent movement. The transverse f issures acted as c i rculat ion channels f o r the mineral izers and considerable amounts of minerals were deposited in them. Thus, the pr incipal Ore shoots are found in the transverse f i ssures . The chief minerals of these f issures are galena, s ide r i t e (manganiferous) f r e i b e r g i t e and sphaler i te . Both the gentlemen previously mentioned noted a ruj.e:- "Where the transverse f au l t taps a longitudinal (100) fault and passes upward out of a hard stratum, such as quartzlte or greenstone, into schist, an ore-shoot is usually found in the vein beneath the schist as i f the latter had acted as an impervious barrier to the ore-bearing solutions and had forced deposition at that ( 1 ) point." Stockwell states, "This is probably due to the fact that the fissure through the harder rocks re-mained open to the ore-bearing solutions, whereas in the schist the fault was more or less sealed by a clayey impervious gouge, forming a dam which forced deposition (2) below i t . " Evidence is offered by Cockfleld to prove the latter . The transverse fissures are short and are not likely to continue with depth but there i s always the possibility of finding ore-bodies in them. The schist however i s not barren of ore in a l l cases. The mineralogy of the veins i s interesting as i t affords some light on the occurrence of the s i lver . Native elements. s i lver , gold. Sulfides. argentite, galena, sphalerite, covell ite, chalcopyrite, pyrite and arsenopyrite. Sulpho-salts pyrargyrite, f re iber -gite, polybaslte, jamesonite. Oxides — quartz, llmonlte, manganlte. Carbonates siderite, calcite, cerussite, malachite, azurite. Sulphates. barite. t l ) Stockwell Op. c i t . (2) Cockfleld W.E. Op. c i t . M'S-.^  t (101) Argentite* Salena - this mineral is uncommon bat la found in small masses enclosed in cerussite and as crystals with galena. - this Is the most important mineral and i s found in nearly every deposit. I t is commonly coarsely crystalline and is not intimately intergrown with the other ore or gangue minerals* The coarse galena has a rather gneiasoid appearance* The fine-grained steel galena la rare but carries average values in si lver* Sphalerite - - - occurs in most of the deposits* I t is generally yellowish brown and resinous in appearance. Covellite — i s very rare. Chalcopyrite-Pyrite —-Arsene pyrites-i s not common and where found i s , as a rule, intimately associated with galena. la f a i r l y abundant and occurs both with arsenopyrite and galena. occurs with quartz and pyrite in the veins of the older series. Pyrorgyrite i s rare. I t occurs with feibergite and galena and was noted in only a few deposits where i t i s of local occurrence and not disseminated. Freibergite — is common and is one of the chief s i lver minerals. Where freibergite i s present even in small quantities the s i lver value of the ore la increased. This mineral ia associated with siderite, galena and sphalerite. Polybasite Jamesonite i s of rare occurrence* be secondary* I t i s believed to also rare. I t may belong to the quartz-arsenopyrite stage of mineralization. Quartz (102) Limonite and Manganite. these are oxidation products. The latter la believed to have been derived from the aider!te* Siderlte or Mangano-alderlte. this i s the moat abundant gangue mineral. The colour varies from dark brown to l ight brown in depth* I t la as a whole finely crystalline, bat many eoaraely crystalline masses were found. This mineral ia always accompanied by freiberglte, galena and sphalerite. Calcite Cercasite ia not common and is mostly associated with aiderlte. ia confined to the surface and is not common* I t occurs as white earthy masses* Malachite and Azurlte --occur as oxidation products. Borite — ia of rare occurrence and ia a gangue mineral* The majority of the veins represent a simple f i l l i n g of fault fissures* Replacement of the wall rock operated only to a alight extent. The ore minerals are in most cases fastened to the polished walls and do net (1) project into them. I t i s believed that the small open-ings of the faults increased as mineralisation took place. I t i s possible that the crystallizing force of the minerals may have assisted the widening of the fissures* Seme replacement i s noted in the Sadie-Treadwell vein of Kens B i l l * (1) docMie ld W.K. d .S.C. Sum* Rp. 1365 A. (110) (1) Cockfield found that the acid dyke rocks, in the Keno Hi l l area, carry small amounts of galena, pyrite and tetrahedrite. These are not considered as the source of the mineralizing solutions but i t is be-lieved that the solutions and the acid dykes have been derived from a larger body of magma. Granitic rocks occur in the vicinity of nearly a l l the ore deposits and I t i s possible that these outliers represent peaks of a batholith that underlies the area. The age of the granite i s not definitely fixed but the ore deposits are younger than the granite, and beyond this their age cannot be fixed more definitely. Secondary enrichment i s practically unknown in glaciated areas but occurs at the St, Eugene, Premier and Dolly Varden mines of British Columbia, In the workings at Keno H i l l area, the frost extended down to the 350 foot level so any secondary enrichment found in this region i s considered to antedote glaciatlon. The frost is believed to have originated in the Pleistocene Period. The geological evidence and mineralogical com-position of the main ore-bodies point to a primary (2) origin and Cockfield i s of the opinion that secondary enrichment played a relatively minor role in the form_ ation of these deposits. (1) Cockfield W.E+ Op. c l t . (2) Cockfield W.E. G.S.C. Sum. Rp. 1923 Pt. A. (104) The silver-antimony veins have a quartz gangue. They carry low values in si lver and their origin i s considered the same as the veins above. All these lead-silver and silver-antimony vein deposits are believed to have been formed at moderate depths by hot ascending solutions. I t is also consider-ed that the mineralizing solutions had their origin in the same magma that gave rise to the acid dykes and si l ls ,and that the changes in ore with depth wi l l depend upon changes in primary deposition. Cockfield notes, I t may also be pointed out that the majority of the ore-bodies occur in connection with outlying bodies of granite rather than with the main mass of the batholith, although there are many ex-ceptions. I t may also be laid down as a general rule that the main mass of the batholith away from its borders is not l ikely to have been the seat of ore deposition, except possible where there are inclusions of the older intruded rocks." These mineral deposits are in many ways similar to the ore deposits of British Columbia. The contact metamorphic deposits are identical. The lead-si lver veins resemble those of the Slocan distr ict and those of the Interior Plateau region of British Columbia. The (1) Cockfield W.E. G.S.C. Sum. Rp. 1922 Pt.A. (135) str ik ing feature o f the l a t t e r i s the f a c t that the s i l v e r values l i k e those o f Yukon are int imately assoeia ted with tetrahedr l te and f r e l b e r g i t e . I t i s found both in Yukon and in Br i t i sh Columbia that i f te t rahedr l te or f r e l b e r g i t e are both absent from a deposit the s i l v e r values are low. In Yukon the s i l v e r has proved to be uniformly distr ibuted through the galena when f r e e from gangue. The average s i l v e r value f o r the area i s 200 ounces per ton. I t i s known that s i l v e r may ex i s t i n galena in the form of sub-microscopic par t i c l es or i n solution only up to 0.2 per cent. Under the microscope the galena from Keno H i l l area shows crysta ls o f argen-t i t e and f r e l b e r g i t e intergrown with the galena. Saline incrustations occur at many points ( 1 ) along the Dezadeash River va l l ey and also along Aishihlk r i v e r . This material i s white and occurs around the edges o f lakes and small ponds, being l e f t as a residue a f t e r the evaporation of the water. These sal ine en-crustations also occur at many points as a thin deposit on top of the s o i l . In some of these cases there i s apparently an abundance of stagnant water early in the spring, and the encrustations are l e f t as the water evaporates, They contain hydrated sulphates o f limo and soda, with a small quantity o f magnesium sulphate, (1 ) Cockfield W.E. G.S.C. Sum. Rp. 1923 P t . A. (106) and insoluble argillaceous and organic matter. They are slightly ferrugenous and contain small quantities of chlorides and phosphates. The potassium content is 0.2 to 0+5 per cent K^O. Coal* The coal-bearing formations of Yukon are a l l of either Tertiary or Jura-Cretaceous age. The mineral fuels in the Tertiary beds are classi f ied as l ignites, while those of Jura-Cretaceous age range from high-grade l ignite to anthracite. The Tertiary coal beds are not of very great areal extent, but have a wide distribution. In places these beds apparently constitute remnants of once larger areas now Infolded with older terranes, but in most cases they represent deposits la id down in separate basins of deposition. The foss i l plant remains show that most of them are of fresh-water origin, These beds are correlated with the Kenai series which is generally referred to the upper Eocene. These rocks are In most places l i t t l e disturbed, although locally they have suffered considerable deformation. The rocks of the Kenai series consist of l ight coloured conglomerates and sandstones and light and dark shales an? c lays. Volcanic material is also associated with these sedim-ents in seme places. The coal of Tertiary age is c lassi f ied as l i g n i t e (107) ( 1 ) and Cockfield states that; "These deposits have BO present economic importance, for other deposits of a much better grade at Tantalus are capable of supplying requirements fo r many years to come*" The Tertiary coals have been mined at only three points; 1. C l i f f Creek; 2. Coal creek (tributary of Yukon r iver ; ) 5* Coal creek (tributary of Rock creek.) The known areas of Tertiary coal beds cover a total of 1,450 square miles with this much area again as a favour-able coal horizon. The estimated tonnage of the known coal areas i s 4,690,000,000. metric tons. The period of Tertiary coal deposition of Yukon probably corresponds with the period of Tertiary coal deposition known in British Columbia to the south. The coal seams of Jura-Cretaceous age are closely associated with the Tantalus conglomerate beds. Two horizons have been recognized — an upper horizon occurr-ing near the top of the Tantalus conglomerate, to which belong the seams at Ishe Tantalus mine; and a lower hor i -zon some distance below the conglomerate which contains the seams at the Five Finger mine, These coal bearing beds have been much mere disturbed than those o f Tertiary age* These Jura-Cretac-eous beds are considered as remnants of former extensive ( 1 ) Cockfield i.Ji "Sixty r i l e and L^due Rivers , " C.C.S. 3921. (108) areas which were originally a l l connected but have been reduced by erosion to their present proportion. In considering the distribution of the coal (1 ) Caimos is of the opinion that, in a general way, i t is chiefly to be found in the areas covered by the Tantalus conglomerate* Farther he declares, i t would be quite possible for coal seams of the lower horizon to be found where the overlying conglomerate was not to bo seen, being either covered by other formations or deposits, or,having been eroded away. In only one locality namely, at the Five Fingers mine have coal seams of any economic value been found where the Tantalus conglomerates are not in evidence. In any ease, Cairnes states, the Coal Measures of the upper horizon contain much the more valuable coal seams. In a l l geological and prospecting work these conglomerates form a very valuable horizon marker, which i s very read-i ly identified, and, when found, the approximate posit-ions of both coal horizons can be determined at once. The thickness of the coal seams of the Tantalus beds varies from eighteen inches up to eight feet . The (2) coal on a whole is classed as low carbon bituminous although individual outcrops vary from lignite to an-thracite. (1) Cairnes D.D. "Lewes and Nordenskiold Rivers," G.S.C. Mem 5, P. 48 1910. (2) Cowling D.B. "Coal Fields of Canada," G.S.C. Mem.59, P. 150, 1915. (109) The only coal mine working these coals i s the Tantalus mine. Here the coal outcrops in three seams on the r i v e r banks and i s , there fore , wel l situated f o r economical working* The three seams have been opened up, but only the lower two have been worked to any extent. The seams are somewhat var iable in width but have averaged 7*6** , 6*6'* and three f e e t o f coal in the bottom, middle and top seams respect ive ly . The low-er two seams have in places not nore than four f e e t o f rock between them, and the middle and top seams are o o general ly reven f e e t apart. The seams dip from 24 to 40 and are somewhat d i r ty but the eoal could eas i l y be washed and stored. An average analysis o f the seams shows the fo l lowing r e su l t s : -Water 0.75 - 0.82 Vo l a t i l e combustible master 25.61 - 25.12 Fixed Carbon 55.21 - 66.05 Ash 20.45 - 8.05 IS535S Firm coherent coke per eent 75.64 - 74.06 The demand f o r coal in th is region i s small and i t i s used mainly f o r fue l ing some o f the r i v e r boats. Accessible wood, near the r i v e r s , used f o r fue l ing the steamers i s rapidly decreasing and e i ther coal or o i l w i l l be required as f u e l in the future . ( i i ) BIBLIOGRAPHY. Brooks, A.H. 1* "The Geography and Geology of Alaska." United States Geological Survey. Professional paper NO.45, 1916. 2+ "A reconnaissance from Pyramid Harbor to Eagle C i ty , Alaska." United States Geological Survey. 21st Annual Report Part I I . 1899 - 1900. Brooks, A.H. and Kindle, E.M. 1. "Palaeozoic an^ associated rocks o f the Upper Yukon, Alaska.* Bul le t in Geological Society of America Volume XIX, 1908. Pages 264 - 271. Caimes, D.D. 1. "Report on a portion of the Conrad and White-horse' Mining D i s t r i c t s , Yukon." Geological Survey of Canada. No. 982B 1908. 2. "Explorations in a portion of the Yukon south o f Whltehorse." C,G.S. Sum. Rp. 1906 Pages 22 - 30. 5. "Recent developements in mining in the southern Yukon. Can. Min. Jour. 28 (n . s l ) pages 87-88, 121 - 122, 1907. 4. "Preliminary memoir on the Lewes and NordenskiSld Rivers Coal D i s t r i c t , Yukon T e r r i t o r y . " Geological Survey o f Canada Memoir 5, 1910. 5. "Wheaton D i s t r i c t , Yukon." Geological Survey of Canada Memoir 51, 1912. 6. "The Yukon - Alaska International Boundary be-tween Porcupine and Yukon Rivers . * Geological Survey of Canada Memoir 67, 1914. 7 . "Upper White River D i s t r i c t . " Geological Survey o f Canada. Mwoi r 50, 1915. 8 . "Portions of the A t l in Mining D i s t r i c t , Br i t i sh Columbia." Geological Survey of Canada. Memoir 57, 1913. 9. "Scroggie, Barker, This t le and Kirkmon Creeks. Geological Survey o f Canada. Memoir 97, 1917. 10. "Explorations in Southern Yukon." Geological Survey o f Canada, Summary Report, 1914. Pages 10 - 33. 11. "Whitehorse and Tantalus Areas." Geological Survey of Canada. Summary Report, 1908. Pages 26 - 28. ( i i i ) 12. "Mayo Area: and Scroggie, Barker, Thistle and Kirk-man greeks; and Wheaton Distr ict . " Geological Survey of Canada, Summary Report, 1915. Pages 40 - 49. 15. Guide Book NO. 10 Geological Survey of Canada, 1913. Pages 51 - 104. 14. "Hansen District . " Geological Survey of Canada, Summary Report, 1914. 15. "Klotassin River Area." Geological Survey of Canada, Summary Report, 1916. Pages 26 - 28S. 16. "Preliminary report on a portion of the Yukon Ter r i t o ry , west of the Lewes r i v e r and between the Latitudes of Whitehorse and Tantalus." Geological Survey of Canada, nummary Report, 1908, Pages 26 - 32. Abst. M. Sc. Press. 9§; 29 - 50 1909. 17. "Wheaton River District^ Yukon Territory." G.S.C- Sum. Rp. 1909, pages 47 - SO 1910. 18. " Geology of a portion r.fthe Yukon -Alaska Boundary, between Porcupine and Yukon Rivers." G.S.C. Sum. Rp. 1911 Pages 17 -35 19. "Quartz Mining in the Klondike D i s t r i c t . " G.S.C. Sum . Rp. 1911 Pages 33 - 40 20. "D i f f e r en t i a l Erosion and Bquiplanation in Portions of Yukon and Alaska." G. Soc. Am. Bul l . 23 Pages 333 - 348. 1912, 21. "Geology of a Portion of the Yukon-Alaska Boundary between Porcupine and Yukon Rivers . " G.S.C. Sum . Rp. 1912, Pages 9 - 11. 22. "Upper White River District, Yukon." G.S.C. Sum. Rp. 1913 Pages 12 - 28. 23. "Some suggested new physiographic terms (equiplana-tlon, deplanation, and applanation.) Am.J. Sc. (4) 34 Pages 75 - 87 1912 Camsell , C. 1. "Wind and Peel Rivers." G.S.C. Sum. Rp. 1904 Vol XVI. Rp. C.C. 2. "Peel River and Tributaries." 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Sum. Rp. 1922, Pt . A. Pages 1 - 8. 12. "Recent Mining Developements in the Msyo D i s t r i c t , Yukon." Can. Min. Jour. Vol 44, No. 50, Pages 980 - 981. Dec. 14 1923. 13. "Geology and Cre deposits in the Keno H i l l Mavo D i s t r i c t , Yukon." G.S.C. Sum. Rp. 1923. Pt . A. 14. "Si lver-Lead Deposits o f Beaver River Area, Yukon." G.S.C. Sum. Rp. 1923 P t . A. Pages 22 - 28. 15. "The Mayo Si lver-Lead D i s t r i c t , Yukon." Can. Min. Jour. Vol 45, No 37, Page 891 - 893, Sept. 12, 1924. Min. Mag. Vo l . 30, No. 2 Pages 122 - 123 Feb. 1924. (cxx) Coekfield W.E. 16* "Upper Beaver River Area, Yukon." G.S.C. Sum.Rp. 1924, Pt. A. 17* "Alshihik Lake District, Yukon." 8+8.C. Sum. Rp. 1926, Pt. A. 18 "Silver-Lead Deposits of Rude Creek, Yukon." G.S.C. Sum.Rp. 1927. Pt. A. 19 "Dezadeash Lake Area." G.S.C* Sum. Rp. 1927 Pt. A. 20 "Silver-Lead Deposit of Fifteen Mile Creek, Yukon." G.S.C. Sum.Rp. 1927 Pt. A. 21 "Pueblo, Tamarack-Carlisle and War-Eagle-Le Roi Properties, Yukon." G.S.C. 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